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) {
    }

}