getLoginStatusUrlString example

  protected function doTestLogin($format) {
    $client = \Drupal::httpClient();
    // Create new user for each iteration to reset flood.     // Grant the user administer users permissions to they can see the     // 'roles' field.     $account = $this->drupalCreateUser(['administer users']);
    $name = $account->getAccountName();
    $pass = $account->passRaw;

    $login_status_url = $this->getLoginStatusUrlString($format);
    $response = $client->get($login_status_url);
    $this->assertHttpResponse($response, 200, UserAuthenticationController::LOGGED_OUT);

    // Flooded.     $this->config('user.flood')
      ->set('user_limit', 3)
      ->save();

    $response = $this->loginRequest($name, 'wrong-pass', $format);
    $this->assertHttpResponseWithMessage($response, 400, 'Sorry, unrecognized username or password.', $format);