.../Core/Test/RunTests/RunTestsResultsBuilder.php  | 245 ---------------------
 core/modules/simpletest/simpletest.module          |  18 +-
 .../simpletest/src/Form/SimpletestResultsForm.php  | 133 ++++++++++-
 core/scripts/run-tests.sh                          |  12 +-
 4 files changed, 147 insertions(+), 261 deletions(-)

diff --git a/core/lib/Drupal/Core/Test/RunTests/RunTestsResultsBuilder.php b/core/lib/Drupal/Core/Test/RunTests/RunTestsResultsBuilder.php
deleted file mode 100644
index b5685edae5..0000000000
--- a/core/lib/Drupal/Core/Test/RunTests/RunTestsResultsBuilder.php
+++ /dev/null
@@ -1,245 +0,0 @@
-<?php
-
-namespace Drupal\Core\Test\RunTests;
-
-use Drupal\Core\Test\TestDiscovery;
-
-/**
- * Build the results part of a test result report.
- *
- * @internal
- */
-class RunTestsResultsBuilder {
-
-  /**
-   * The inline Twig template for a results summary.
-   *
-   * @var string
-   */
-  protected static $summaryTemplate = <<<'TWIG'
-<div class="simpletest-{{ fail + exception == 0 ? 'pass' : 'fail' }}">
-  {{ label }} {{ items|join(', ') }}
-</div>
-TWIG;
-
-  /**
-   * Builds the status image map.
-   *
-   * @return string[]
-   *   Render array of icons for status displays.
-   */
-  public static function buildStatusImageMap() {
-    $image_pass = [
-      '#theme' => 'image',
-      '#uri' => 'core/misc/icons/73b355/check.svg',
-      '#width' => 18,
-      '#height' => 18,
-      '#alt' => 'Pass',
-    ];
-    $image_fail = [
-      '#theme' => 'image',
-      '#uri' => 'core/misc/icons/e32700/error.svg',
-      '#width' => 18,
-      '#height' => 18,
-      '#alt' => 'Fail',
-    ];
-    $image_exception = [
-      '#theme' => 'image',
-      '#uri' => 'core/misc/icons/e29700/warning.svg',
-      '#width' => 18,
-      '#height' => 18,
-      '#alt' => 'Exception',
-    ];
-    $image_debug = [
-      '#theme' => 'image',
-      '#uri' => 'core/misc/icons/e29700/warning.svg',
-      '#width' => 18,
-      '#height' => 18,
-      '#alt' => 'Debug',
-    ];
-    return [
-      'pass' => $image_pass,
-      'fail' => $image_fail,
-      'exception' => $image_exception,
-      'debug' => $image_debug,
-    ];
-  }
-
-  /**
-   * Adds the result form to a $form.
-   *
-   * This is a static method so that run-tests.sh can use it to generate a
-   * results page completely external to Drupal. This is why the UI strings are
-   * not wrapped in t().
-   *
-   * @param array $form
-   *   The form to attach the results to.
-   * @param array $results
-   *   The simpletest results.
-   *
-   * @return array
-   *   A list of tests the passed and failed. The array has two keys, 'pass' and
-   *   'fail'. Each contains a list of test classes.
-   *
-   * @see simpletest_script_open_browser()
-   * @see run-tests.sh
-   * @see \Drupal\simpletest\Form\SimpletestResultsForm
-   */
-  public static function addResultForm(array &$form, array $results) {
-    // Transform the test results to be grouped by test class.
-    $test_results = [];
-    foreach ($results as $result) {
-      if (!isset($test_results[$result->test_class])) {
-        $test_results[$result->test_class] = [];
-      }
-      $test_results[$result->test_class][] = $result;
-    }
-
-    $image_status_map = static::buildStatusImageMap();
-
-    // Keep track of which test cases passed or failed.
-    $filter = [
-      'pass' => [],
-      'fail' => [],
-    ];
-
-    // Summary result widget.
-    $form['result'] = [
-      '#type' => 'fieldset',
-      '#title' => 'Results',
-      // Because this is used in a theme-less situation need to provide a
-      // default.
-      '#attributes' => [],
-    ];
-
-    // Add summary line.
-    $form['result']['summary'] = $summary = [
-      '#type' => 'inline_template',
-      '#template' => static::$summaryTemplate,
-      // Fill in the context for the template after we've tallied results in
-      // #pass, #fail, etc.
-      '#context' => [],
-      '#pass' => 0,
-      '#fail' => 0,
-      '#exception' => 0,
-      '#debug' => 0,
-    ];
-
-    \Drupal::service('test_discovery')->registerTestNamespaces();
-
-    // Cycle through each test group.
-    $header = [
-      'Message',
-      'Group',
-      'Filename',
-      'Line',
-      'Function',
-      ['colspan' => 2, 'data' => 'Status'],
-    ];
-    $form['result']['results'] = [];
-    foreach ($test_results as $group => $assertions) {
-      // Create group details with summary information.
-      $info = TestDiscovery::getTestInfo($group);
-      $form['result']['results'][$group] = [
-        '#type' => 'details',
-        '#title' => $info['name'],
-        '#open' => TRUE,
-        '#description' => $info['description'],
-      ];
-      $form['result']['results'][$group]['summary'] = $summary;
-      $group_summary = & $form['result']['results'][$group]['summary'];
-
-      // Create table of assertions for the group.
-      $rows = [];
-      foreach ($assertions as $assertion) {
-        $row = [];
-        $row[] = ['data' => ['#markup' => $assertion->message]];
-        $row[] = $assertion->message_group;
-        $row[] = \Drupal::service('file_system')->basename(($assertion->file));
-        $row[] = $assertion->line;
-        $row[] = $assertion->function;
-        $row[] = ['data' => $image_status_map[$assertion->status]];
-
-        $class = 'simpletest-' . $assertion->status;
-        if ($assertion->message_group == 'Debug') {
-          $class = 'simpletest-debug';
-        }
-        $rows[] = ['data' => $row, 'class' => [$class]];
-
-        $group_summary['#' . $assertion->status]++;
-        $form['result']['summary']['#' . $assertion->status]++;
-      }
-      $form['result']['results'][$group]['table'] = [
-        '#type' => 'table',
-        '#header' => $header,
-        '#rows' => $rows,
-      ];
-
-      $group_summary['#context'] = static::buildSummaryContext(
-        $group_summary['#pass'],
-        $group_summary['#fail'],
-        $group_summary['#exception'],
-        $group_summary['#debug']
-      );
-
-      // Set summary information.
-      $group_summary['#ok'] = $group_summary['#fail'] + $group_summary['#exception'] == 0;
-      $form['result']['results'][$group]['#open'] = !$group_summary['#ok'];
-
-      // Store test group (class) as for use in filter.
-      $filter[$group_summary['#ok'] ? 'pass' : 'fail'][] = $group;
-    }
-
-    // Now that we've tallied all the results, we can compute the context for
-    // the results summary template.
-
-    $form['result']['summary']['#context'] = static::buildSummaryContext(
-      $form['result']['summary']['#pass'],
-      $form['result']['summary']['#fail'],
-      $form['result']['summary']['#exception'],
-      $form['result']['summary']['#debug']
-    );
-
-    // Overall summary status.
-    $form['result']['summary']['#ok'] = $form['result']['summary']['#fail'] + $form['result']['summary']['#exception'] == 0;
-
-    return $filter;
-  }
-
-  /**
-   * Assemble the context for the twig template in static::$summaryTemplate.
-   *
-   * @param int $pass
-   *   The count of passing tests.
-   * @param int $fail
-   *   The count of failing tests.
-   * @param int $exception
-   *   The count of tests that threw an exception.
-   * @param int $debug
-   *   The count of debug messages present.
-   * @param string $label
-   *   An optional label for the summary line.
-   *
-   * @return string[]
-   *   The context for the twig template in static::$summaryTemplate.
-   */
-  protected static function buildSummaryContext($pass, $fail, $exception, $debug, $label = NULL) {
-    $context = [];
-    $translation = \Drupal::translation();
-
-    $context['label'] = $label;
-    $context['pass'] = $pass;
-    $context['fail'] = $fail;
-    $context['exception'] = $exception;
-    $context['debug'] = $debug;
-
-    $context['items']['pass'] = $translation->formatPlural($pass, '1 pass', '@count passes');
-    $context['items']['fail'] = $translation->formatPlural($fail, '1 fail', '@count fails');
-    $context['items']['exception'] = $translation->formatPlural($exception, '1 exception', '@count exceptions');
-    if ($debug) {
-      $context['items']['debug'] = $translation->formatPlural($debug, '1 debug message', '@count debug messages');
-    }
-    return $context;
-  }
-
-}
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 3c6f0feeb6..5775c1d83d 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -41,10 +41,6 @@ function simpletest_help($route_name, RouteMatchInterface $route_match) {
 
 /**
  * Implements hook_theme().
- *
- * @todo Replace this theming system with
- *   Drupal\Core\Test\RunTests\RunTestsResultsBuilder in
- *   https://www.drupal.org/project/drupal/issues/3075490
  */
 function simpletest_theme() {
   return [
@@ -104,6 +100,20 @@ function _simpletest_build_summary_line($summary) {
   return $items;
 }
 
+/**
+ * Formats test result summaries into a comma separated string for run-tests.sh.
+ *
+ * @param array $summary
+ *   A summary of the test results.
+ *
+ * @return string
+ *   A concatenated string of the formatted test results.
+ */
+function _simpletest_format_summary_line($summary) {
+  $parts = _simpletest_build_summary_line($summary);
+  return implode(', ', $parts);
+}
+
 /**
  * Runs tests.
  *
diff --git a/core/modules/simpletest/src/Form/SimpletestResultsForm.php b/core/modules/simpletest/src/Form/SimpletestResultsForm.php
index 5de37fcbbe..adc1d4a5bb 100644
--- a/core/modules/simpletest/src/Form/SimpletestResultsForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestResultsForm.php
@@ -7,8 +7,8 @@
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Test\EnvironmentCleanerInterface;
-use Drupal\Core\Test\RunTests\RunTestsResultsBuilder;
 use Drupal\Core\Url;
+use Drupal\simpletest\TestDiscovery;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -70,7 +70,40 @@ public function __construct(Connection $database, EnvironmentCleanerInterface $c
    * Builds the status image map.
    */
   protected static function buildStatusImageMap() {
-    return RunTestsResultsBuilder::buildStatusImageMap();
+    $image_pass = [
+      '#theme' => 'image',
+      '#uri' => 'core/misc/icons/73b355/check.svg',
+      '#width' => 18,
+      '#height' => 18,
+      '#alt' => 'Pass',
+    ];
+    $image_fail = [
+      '#theme' => 'image',
+      '#uri' => 'core/misc/icons/e32700/error.svg',
+      '#width' => 18,
+      '#height' => 18,
+      '#alt' => 'Fail',
+    ];
+    $image_exception = [
+      '#theme' => 'image',
+      '#uri' => 'core/misc/icons/e29700/warning.svg',
+      '#width' => 18,
+      '#height' => 18,
+      '#alt' => 'Exception',
+    ];
+    $image_debug = [
+      '#theme' => 'image',
+      '#uri' => 'core/misc/icons/e29700/warning.svg',
+      '#width' => 18,
+      '#height' => 18,
+      '#alt' => 'Debug',
+    ];
+    return [
+      'pass' => $image_pass,
+      'fail' => $image_fail,
+      'exception' => $image_exception,
+      'debug' => $image_debug,
+    ];
   }
 
   /**
@@ -224,7 +257,101 @@ protected function getResults($test_id) {
    * @see run-tests.sh
    */
   public static function addResultForm(array &$form, array $results) {
-    return RunTestsResultsBuilder::addResultForm($form, $results);
+    // Transform the test results to be grouped by test class.
+    $test_results = [];
+    foreach ($results as $result) {
+      if (!isset($test_results[$result->test_class])) {
+        $test_results[$result->test_class] = [];
+      }
+      $test_results[$result->test_class][] = $result;
+    }
+
+    $image_status_map = static::buildStatusImageMap();
+
+    // Keep track of which test cases passed or failed.
+    $filter = [
+      'pass' => [],
+      'fail' => [],
+    ];
+
+    // Summary result widget.
+    $form['result'] = [
+      '#type' => 'fieldset',
+      '#title' => 'Results',
+      // Because this is used in a theme-less situation need to provide a
+      // default.
+      '#attributes' => [],
+    ];
+    $form['result']['summary'] = $summary = [
+      '#theme' => 'simpletest_result_summary',
+      '#pass' => 0,
+      '#fail' => 0,
+      '#exception' => 0,
+      '#debug' => 0,
+    ];
+
+    \Drupal::service('test_discovery')->registerTestNamespaces();
+
+    // Cycle through each test group.
+    $header = [
+      'Message',
+      'Group',
+      'Filename',
+      'Line',
+      'Function',
+      ['colspan' => 2, 'data' => 'Status'],
+    ];
+    $form['result']['results'] = [];
+    foreach ($test_results as $group => $assertions) {
+      // Create group details with summary information.
+      $info = TestDiscovery::getTestInfo($group);
+      $form['result']['results'][$group] = [
+        '#type' => 'details',
+        '#title' => $info['name'],
+        '#open' => TRUE,
+        '#description' => $info['description'],
+      ];
+      $form['result']['results'][$group]['summary'] = $summary;
+      $group_summary =& $form['result']['results'][$group]['summary'];
+
+      // Create table of assertions for the group.
+      $rows = [];
+      foreach ($assertions as $assertion) {
+        $row = [];
+        $row[] = ['data' => ['#markup' => $assertion->message]];
+        $row[] = $assertion->message_group;
+        $row[] = \Drupal::service('file_system')->basename(($assertion->file));
+        $row[] = $assertion->line;
+        $row[] = $assertion->function;
+        $row[] = ['data' => $image_status_map[$assertion->status]];
+
+        $class = 'simpletest-' . $assertion->status;
+        if ($assertion->message_group == 'Debug') {
+          $class = 'simpletest-debug';
+        }
+        $rows[] = ['data' => $row, 'class' => [$class]];
+
+        $group_summary['#' . $assertion->status]++;
+        $form['result']['summary']['#' . $assertion->status]++;
+      }
+      $form['result']['results'][$group]['table'] = [
+        '#type' => 'table',
+        '#header' => $header,
+        '#rows' => $rows,
+      ];
+
+      // Set summary information.
+      $group_summary['#ok'] = $group_summary['#fail'] + $group_summary['#exception'] == 0;
+      $form['result']['results'][$group]['#open'] = !$group_summary['#ok'];
+
+      // Store test group (class) as for use in filter.
+      $filter[$group_summary['#ok'] ? 'pass' : 'fail'][] = $group;
+    }
+
+    // Overall summary status.
+    $form['result']['summary']['#ok'] = $form['result']['summary']['#fail'] + $form['result']['summary']['#exception'] == 0;
+
+    return $filter;
   }
 
 }
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 0b40f51e3f..9d3f3894eb 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -16,10 +16,10 @@
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Test\EnvironmentCleaner;
 use Drupal\Core\Test\PhpUnitTestRunner;
-use Drupal\Core\Test\RunTests\RunTestsResultsBuilder;
 use Drupal\Core\Test\RunTests\TestFileParser;
 use Drupal\Core\Test\TestDatabase;
 use Drupal\Core\Test\TestRunnerKernel;
+use Drupal\simpletest\Form\SimpletestResultsForm;
 use Drupal\Core\Test\TestDiscovery;
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Console\Output\ConsoleOutput;
@@ -815,9 +815,6 @@ function simpletest_script_run_one_test($test_id, $test_class) {
   global $args;
 
   try {
-    // Default to status = success. This could mean that we didn't discover any
-    // tests and that none ran.
-    $status = SIMPLETEST_SCRIPT_EXIT_SUCCESS;
     if (strpos($test_class, '::') > 0) {
       list($class_name, $method) = explode('::', $test_class, 2);
       $methods = [$method];
@@ -834,10 +831,7 @@ function simpletest_script_run_one_test($test_id, $test_class) {
     if (is_subclass_of($test_class, TestCase::class)) {
       $status = simpletest_script_run_phpunit($test_id, $test_class);
     }
-    // If we aren't running a PHPUnit-based test, then we might have a
-    // Simpletest-based one. Ensure that: 1) The simpletest framework exists,
-    // and 2) That our test belongs to that framework.
-    elseif (class_exists('\Drupal\simpletest\TestBase') && is_subclass_of($test_class, '\Drupal\simpletest\TestBase')) {
+    else {
       $test->dieOnFail = (bool) $args['die-on-fail'];
       $test->verbose = (bool) $args['verbose'];
       $test->run($methods);
@@ -1521,7 +1515,7 @@ function simpletest_script_open_browser() {
 
   // Get the results form.
   $form = [];
-  RunTestsResultsBuilder::addResultForm($form, $results);
+  SimpletestResultsForm::addResultForm($form, $results);
 
   // Get the assets to make the details element collapsible and theme the result
   // form.