diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index 1b512b5..4ab827d 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -207,7 +207,8 @@ public function findAllClassFiles($extension = NULL) {
     $classmap = array();
     $namespaces = $this->registerTestNamespaces();
     if (isset($extension)) {
-      $namespaces = array_intersect_key($namespaces, array_flip(preg_grep('/' . preg_quote("Drupal\\$extension\\", '/') . '/', array_keys($namespaces))));
+      $pattern = '/' . preg_quote("Drupal\\", '/') . '(Tests\\\)?' . preg_quote("$extension\\", '/') . '/';
+      $namespaces = array_intersect_key($namespaces, array_flip(preg_grep($pattern, array_keys($namespaces))));
     }
     foreach ($namespaces as $namespace => $paths) {
       foreach ($paths as $path) {