Issue #588438 by dww: FormTest::testRequiredFields() fails to drupal_render()...

          \Drupal::formBuilder()->prepareForm($form_id, $form, $form_state);

          \Drupal::formBuilder()->processForm($form_id, $form, $form_state);

          $errors = $form_state->getErrors();

          // Form elements of type 'radios' throw all sorts of PHP notices

          // when you try to render them like this, so we ignore those for

          // testing the required marker.

          // @todo Fix this work-around (https://www.drupal.org/node/588438).

          $form_output = ($type == 'radios') ? '' : \Drupal::service('renderer')->renderRoot($form);

          $form_output = \Drupal::service('renderer')->renderRoot($form);

          if ($required) {

            // Make sure we have a form error for this element.

            $this->assertTrue(isset($errors[$element]), "Check empty($key) '$type' field '$element'");