StubForm example

foreach ([TRUE, FALSE] as $required) {
          $form_id = $this->randomMachineName();
          $form = [];
          $form_state = new FormState();
          $form['op'] = ['#type' => 'submit', '#value' => 'Submit'];
          $element = $data['element']['#title'];
          $form[$element] = $data['element'];
          $form[$element]['#required'] = $required;
          $user_input[$element] = $empty;
          $user_input['form_id'] = $form_id;
          $form_state->setUserInput($user_input);
          $form_state->setFormObject(new StubForm($form_id, $form));
          $form_state->setMethod('POST');
          // The form token CSRF protection should not interfere with this test,           // so we bypass it by setting the token to FALSE.           $form['#token'] = FALSE;
          \Drupal::formBuilder()->prepareForm($form_id, $form, $form_state);
          \Drupal::formBuilder()->processForm($form_id, $form, $form_state);
          $errors = $form_state->getErrors();
          $form_output = \Drupal::service('renderer')->renderRoot($form);
          if ($required) {
            // Make sure we have a form error for this element.             $this->assertTrue(isset($errors[$element]), "Check empty($key) '$type' field '$element'");
            

// The form token CSRF protection should not interfere with this test, so we     // bypass it by setting the token to FALSE.     $form['#token'] = FALSE;

    $edit['form_id'] = $form_id;

    // Disable page redirect for forms submitted programmatically. This is a     // solution to skip the redirect step (there are no pages, then the redirect     // isn't possible).     $form_state->disableRedirect();
    $form_state->setUserInput($edit);
    $form_state->setFormObject(new StubForm($form_id, $form));

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

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

    $errors = $form_state->getErrors();

    // Clear errors and messages.     \Drupal::messenger()->deleteAll();
    $form_state->clearErrors();