ClassyTest.php

<?php

namespace Drupal\Tests\classy\Kernel;

use Drupal\KernelTests\KernelTestBase;

/**
 * Tests the Classy theme.
 *
 * @group classy
 * @group legacy
 */
class ClassyTest extends KernelTestBase {
  
  /**
   * {@inheritdoc}
   */
  protected static $modules = [
    'system',
    'twig_theme_test',
  ];
  
  /**
   * {@inheritdoc}
   */
  public function setUp() : void {
    parent::setUp();
    // Use the classy theme.
    $this->container
      ->get('theme_installer')
      ->install([
      'classy',
    ]);
    $this->container
      ->get('config.factory')
      ->getEditable('system.theme')
      ->set('default', 'classy')
      ->save();
    // Clear the theme registry.
    $this->container
      ->set('theme.registry', NULL);
  }
  
  /**
   * Tests the classy theme.
   */
  public function testClassyTheme() {
    \Drupal::messenger()->addError('An error occurred');
    \Drupal::messenger()->addStatus('But then something nice happened');
    $messages = [
      '#type' => 'status_messages',
    ];
    $this->render($messages);
    $this->assertNoText('custom-test-messages-class', 'The custom class attribute value added in the status messages preprocess function is not displayed as page content.');
  }

}