TestMultipleFormController.php

<?php

namespace Drupal\block_test\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Form\FormState;

/**
 * Controller for block_test module.
 */
class TestMultipleFormController extends ControllerBase {
    public function testMultipleForms() {
        $form_state = new FormState();
        $build = [
            'form1' => $this->formBuilder()
                ->buildForm('\\Drupal\\block_test\\Form\\TestForm', $form_state),
            'form2' => $this->formBuilder()
                ->buildForm('\\Drupal\\block_test\\Form\\FavoriteAnimalTestForm', $form_state),
        ];
        // Output all attached placeholders trough
        // \Drupal\Core\Messenger\MessengerInterface::addMessage(), so we can
        // see if there's only one in the tests.
        $post_render_callable = function ($elements) {
            $matches = [];
            preg_match_all('<form\\s(.*?)action="(.*?)"(.*)>', $elements, $matches);
            $action_values = $matches[2];
            foreach ($action_values as $action_value) {
                $this->messenger()
                    ->addStatus('Form action: ' . $action_value);
            }
            return $elements;
        };
        $build['#post_render'] = [
            $post_render_callable,
        ];
        return $build;
    }

}