TestForm.php

<?php

namespace Drupal\page_cache_form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * A form to test page cache.
 *
 * @internal
 */
class TestForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'page_cache_form_test';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['#prefix'] = '<p>Llamas are awesome, but kittens are pretty cool too!</p>';
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  }

}