Michael Strelan
--- a/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
        'tags' => [['name' => 'event_subscriber']],
      ];
    }
+    // Register test middleware.
+    $services['services']['testing.http_client.middleware'] = [
+      'class' => 'Drupal\Core\Test\HttpClientMiddleware\TestHttpClientMiddleware',
+      'tags' => [['name' => 'http_client_middleware']],
+    ];
+    $services['services']['testing.http_middleware.wait_terminate_middleware'] = [
+      'class' => 'Drupal\Core\Test\StackMiddleware\TestWaitTerminateMiddleware',
+      'arguments' => ['@state', '@lock'],
+      'tags' => [['name' => 'http_middleware', 'priority' => -1024]],
+    ];
--- a/core/lib/Drupal/Core/Test/StackMiddleware/TestWaitTerminateMiddleware.php
+++ b/core/lib/Drupal/Core/Test/StackMiddleware/TestWaitTerminateMiddleware.php
  public function handle(Request $request, $type = self::MAIN_REQUEST, $catch = TRUE): Response {
    $result = $this->httpKernel->handle($request, $type, $catch);

-    if ($this->state->get('drupal.test_wait_terminate')) {
-      // Set a header on the response to instruct the test runner that it must
-      // await the lock. Note that the lock acquired here is automatically
-      // released from within a shutdown function.
-      $this->lock->acquire('test_wait_terminate');
-      $result->headers->set('X-Drupal-Wait-Terminate', '1');
+    if (!$this->waitForTerminate) {
+      return $result;
--- a/core/lib/Drupal/Core/CoreServiceProvider.php
+++ b/core/lib/Drupal/Core/CoreServiceProvider.php
   * {@inheritdoc}
   */
  public function register(ContainerBuilder $container) {
-    $this->registerTest($container);
-