diff --git a/core/modules/simpletest/src/Form/SimpletestTestForm.php b/core/modules/simpletest/src/Form/SimpletestTestForm.php
index 488a093..ae185f1 100644
--- a/core/modules/simpletest/src/Form/SimpletestTestForm.php
+++ b/core/modules/simpletest/src/Form/SimpletestTestForm.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\RendererInterface;
+use Drupal\simpletest\TestDiscovery;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -31,11 +32,19 @@ class SimpletestTestForm extends FormBase {
   protected $appRoot;
 
   /**
+   * The test discovery service.
+   *
+   * @var \Drupal\simpletest\TestDiscovery
+   */
+  protected $testDiscovery;
+
+  /**
    * {@inheritdoc}
    */
   public static function create(ContainerInterface $container) {
     return new static(
       $container->get('renderer'),
+      $container->get('test_discovery'),
       (string) $container->get('app.root')
     );
   }
@@ -48,8 +57,9 @@ public static function create(ContainerInterface $container) {
    * @param \Drupal\simpletest\TestDiscovery $test_discovery
    *   The test discovery service.
    */
-  public function __construct(RendererInterface $renderer, $app_root) {
+  public function __construct(RendererInterface $renderer, TestDiscovery $test_discovery, $app_root) {
     $this->renderer = $renderer;
+    $this->testDiscovery = $test_discovery;
     $this->appRoot = $app_root;
   }
 
@@ -158,11 +168,11 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       $extension_names[$type] = array_keys($extension_discovery->scan($type, TRUE));
     }
     // Generate the list of tests arranged by group.
-    $groups = simpletest_test_get_all();
+    $groups = $this->testDiscovery->getTestClasses();
     foreach ($groups as $group => $tests) {
       // Add the extension type to group names which are also extension names.
       foreach ($extension_types as $type) {
-        if (in_array($group, $extension_names[$type])) {
+        if (in_array($group, $extension_names[$type], TRUE)) {
           $group = $group . ' ' . $type;
         }
       }
@@ -229,7 +239,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
     // Test discovery does not run upon form submission.
-    simpletest_classloader_register();
+    $this->testDiscovery->registerTestNamespaces();
 
     // This form accepts arbitrary user input for 'tests'.
     // An invalid value will cause the $class_name lookup below to die with a