Issue #2747273 by quietone, mayurjadhav, talhaparacha: Example given on...

   *

   * For example:

   * @code

   * // register a new user

   * // Set the administrator role to 'content_editor'.

   * $values['user_admin_role'] = 'content_editor';

   * $form_state = new FormState();

   * $values['name'] = 'some-user';

   * $values['mail'] = 'someuser@example.com';

   * $values['pass']['pass1'] = 'password';

   * $values['pass']['pass2'] = 'password';

   * $values['op'] = t('Create new account');

   * $form_state->setValues($values);

   * \Drupal::formBuilder()->submitForm('user_register_form', $form_state);

   * \Drupal::formBuilder()->submitForm(RoleSettingsForm::class, $form_state);

   * @endcode

   *

   * @param \Drupal\Core\Form\FormInterface|string $form_arg