TestHttpClientMiddleware example

->addArgument('%language.default_values%');
    $this->container
      ->register('string_translation', 'Drupal\Core\StringTranslation\TranslationManager')
      ->addArgument(new Reference('language.default'));
    $this->container
      ->register('http_client', 'GuzzleHttp\Client')
      ->setFactory('http_client_factory:fromOptions');
    $this->container
      ->register('http_client_factory', 'Drupal\Core\Http\ClientFactory')
      ->setArguments([new Reference('http_handler_stack')]);
    $handler_stack = HandlerStack::create();
    $test_http_client_middleware = new TestHttpClientMiddleware();
    $handler_stack->push($test_http_client_middleware(), 'test.http_client.middleware');
    $this->container
      ->set('http_handler_stack', $handler_stack);

    $this->container
      ->setParameter('app.root', DRUPAL_ROOT);
    \Drupal::setContainer($this->container);

    // Setup Mink.     $this->initMink();