diff --git a/core/modules/simpletest/src/Form/SimpletestTestForm.php b/core/modules/simpletest/src/Form/SimpletestTestForm.php
index ec99784..3cac3f4 100644
--- a/core/modules/simpletest/src/Form/SimpletestTestForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestTestForm.php
@@ -2,10 +2,10 @@
 
 namespace Drupal\simpletest\Form;
 
+use Drupal\Core\Extension\ExtensionDiscovery;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\RendererInterface;
-use Drupal\simpletest\TestDiscovery;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -21,11 +21,14 @@ class SimpletestTestForm extends FormBase {
   protected $renderer;
 
   /**
-   * The test discovery service.
+   * Path to the application root.
    *
-   * @var \Drupal\simpletest\TestDiscovery
+   * We need this so that we can do discovery of extensions, and show the user
+   * which tests belong with which extension.
+   *
+   * @var string
    */
-  protected $testDiscovery;
+  protected $appRoot;
 
   /**
    * {@inheritdoc}
@@ -33,7 +36,7 @@ class SimpletestTestForm extends FormBase {
   public static function create(ContainerInterface $container) {
     return new static(
       $container->get('renderer'),
-      $container->get('test_discovery')
+      (string) $container->get('app.root')
     );
   }
 
@@ -43,9 +46,9 @@ public static function create(ContainerInterface $container) {
    * @param \Drupal\Core\Render\RendererInterface $renderer
    *   The renderer.
    */
-  public function __construct(RendererInterface $renderer, TestDiscovery $test_discovery) {
+  public function __construct(RendererInterface $renderer, $app_root) {
     $this->renderer = $renderer;
-    $this->testDiscovery = $test_discovery;
+    $this->appRoot = $app_root;
   }
 
   /**
@@ -145,14 +148,21 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       (string) $this->renderer->renderPlain($image_extended),
     ];
 
+    // Get a list of all available extension names.
+    $extension_discovery = new ExtensionDiscovery($this->appRoot);
+    $extension_names = [];
+    $extension_types = ['module', 'theme', 'profile', 'theme engine'];
+    foreach($extension_types as $type) {
+      $extension_names[$type] = array_keys($extension_discovery->scan($type, TRUE));
+    }
     // Generate the list of tests arranged by group.
-    $groups = $this->testDiscovery->getTestClasses();
+    $groups = simpletest_test_get_all();
     foreach ($groups as $group => $tests) {
-      // The convention is that groups based on namespaces are capitalized,
-      // and groups based on module names are not. In order to provide clarity,
-      // we add 'module' to module group names.
-      if (strtolower($group) == $group) {
-        $group = $group . ' module';
+      // Add the extension type to group names which are also extension names.
+      foreach ($extension_types as $type) {
+        if (in_array($group, $extension_names[$type])) {
+          $group = $group . ' ' . $type;
+        }
       }
 
       $form['tests'][$group] = array(
@@ -217,7 +227,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
     // Test discovery does not run upon form submission.
-    $this->testDiscovery->registerTestNamespaces();
+    simpletest_classloader_register();
 
     // This form accepts arbitrary user input for 'tests'.
     // An invalid value will cause the $class_name lookup below to die with a