diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 627f6dc..690bc8c 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -54,6 +54,13 @@
   protected $databasePrefix = NULL;
 
   /**
+   * The site directory of the original parent site.
+   *
+   * @var string
+   */
+  protected $originalSite;
+
+  /**
    * The original file directory, before it was changed for testing purposes.
    *
    * @var string
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/Tests/SimpleTestTest.php b/core/modules/simpletest/lib/Drupal/simpletest/Tests/SimpleTestTest.php
index e0f949d..492b86f 100755
--- a/core/modules/simpletest/lib/Drupal/simpletest/Tests/SimpleTestTest.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/Tests/SimpleTestTest.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\simpletest\Tests;
 
-use Drupal\Core\Database\Driver\pgsql\Select;
 use Drupal\simpletest\WebTestBase;
 
 /**
@@ -44,7 +43,34 @@ public static function getInfo() {
 
   function setUp() {
     if (!$this->isInChildSite()) {
-      file_put_contents($this->siteDirectory. '/' . 'settings.testing.php', '<?php if (!function_exists("simpletest_test_stub_settings_function")) {function simpletest_test_stub_settings_function() {}}');
+      $php = <<<'EOD'
+<?php
+
+# Make sure that the $test_class variable is defined when this file is included.
+if ($test_class) {
+}
+
+# Define a function to be able to check that this file was loaded with
+# function_exists().
+if (!function_exists('simpletest_test_stub_settings_function')) {
+  function simpletest_test_stub_settings_function() {}
+}
+EOD;
+
+      file_put_contents($this->siteDirectory. '/' . 'settings.testing.php', $php);
+      // @see \Drupal\system\Tests\DrupalKernel\DrupalKernelSiteTest
+      $class = __CLASS__;
+      $yaml = <<<EOD
+services:
+  # Add a new service.
+  site.service.yml:
+    class: $class
+  # Swap out a core service.
+  cache.backend.database:
+    class: Drupal\Core\Cache\MemoryBackendFactory
+EOD;
+      file_put_contents($this->siteDirectory . '/testing.services.yml', $yaml);
+
       parent::setUp();
       // Create and log in an admin user.
       $this->drupalLogin($this->drupalCreateUser(array('administer unit tests')));
@@ -192,11 +218,14 @@ function stubTest() {
     $this->drupalCreateUser(array($this->invalid_permission));
 
     $this->pass(t('Test ID is @id.', array('@id' => $this->testId)));
-    // These two are tested by asserting the right number of passes and
-    // failures.
+    // These four assertions are tested by asserting the right number of
+    // passes and failures.
     $this->assertTrue(file_exists(conf_path() . '/settings.testing.php'));
     // Check the settings.testing.php file got included.
     $this->assertTrue(function_exists('simpletest_test_stub_settings_function'));
+    // Check that the test-specific service file got loaded.
+    $this->assertTrue($this->container->has('site.service.yml'));
+    $this->assertIdentical(get_class($this->container->get('cache.backend.database')), 'Drupal\Core\Cache\MemoryBackendFactory');
 
     // Call trigger_error() without the required argument to trigger an E_WARNING.
     trigger_error();
@@ -243,7 +272,7 @@ function confirmStubTestResults() {
 
     $this->assertAssertion("Debug: 'Foo'", 'Debug', 'Fail', 'SimpleTestTest.php', 'Drupal\simpletest\Tests\SimpleTestTest->stubTest()');
 
-    $this->assertEqual('8 passes, 5 fails, 2 exceptions, 1 debug message', $this->childTestResults['summary']);
+    $this->assertEqual('10 passes, 5 fails, 2 exceptions, 1 debug message', $this->childTestResults['summary']);
 
     $this->test_ids[] = $test_id = $this->getTestIdFromResults();
     $this->assertTrue($test_id, 'Found test ID in results.');
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 1a494dd..963de4a 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -843,14 +843,21 @@ protected function setUp() {
       'required' => TRUE,
     );
     $this->writeSettings($settings);
+
+    // Allow for test-specific overrides.
     $settings_testing_file = DRUPAL_ROOT . '/' . $this->originalSite . '/settings.testing.php';
     if (file_exists($settings_testing_file)) {
-      // Copy the testing specific overrides in place.
+      // Copy the testing-specific overrides in place.
       copy($settings_testing_file, $directory . '/settings.testing.php');
       // Add the name of the testing class to settings.php and include the
       // testing specific overrides
       file_put_contents($directory . '/settings.php', "\n\$test_class = '" . get_class($this) ."';\n" . 'include DRUPAL_ROOT . \'/\' . $conf_path . \'/settings.testing.php\';' ."\n", FILE_APPEND);
     }
+    $settings_services_file = DRUPAL_ROOT . '/' . $this->originalSite . '/testing.services.yml';
+    if (file_exists($settings_services_file)) {
+      // Copy the testing-specific service overrides in place.
+      copy($settings_services_file, $directory . '/services.yml');
+    }
 
     // Since Drupal is bootstrapped already, install_begin_request() will not
     // bootstrap into DRUPAL_BOOTSTRAP_CONFIGURATION (again). Hence, we have to