diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index e81e0ff..2c86907 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -207,6 +207,7 @@ public function findAllClassFiles($extension = NULL) {
     $classmap = array();
     $namespaces = $this->registerTestNamespaces();
     if (isset($extension)) {
+      // Include tests in the \Drupal\Tests\{$extension} namespace.
       $pattern = "/Drupal\\\(Tests\\\)?$extension\\\/";
       $namespaces = array_intersect_key($namespaces, array_flip(preg_grep($pattern, array_keys($namespaces))));
     }