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.');
    }

}