MessageTest.php

<?php

namespace Drupal\KernelTests\Core\Theme;

use Drupal\KernelTests\KernelTestBase;

/**
 * Tests built-in message theme functions.
 *
 * @group Theme
 */
class MessageTest extends KernelTestBase {
    
    /**
     * {@inheritdoc}
     */
    protected static $modules = [
        'system',
    ];
    
    /**
     * Tests setting messages output.
     */
    public function testMessages() {
        // Enable the Starterkit theme.
        \Drupal::service('theme_installer')->install([
            'starterkit_theme',
        ]);
        $this->config('system.theme')
            ->set('default', 'starterkit_theme')
            ->save();
        \Drupal::messenger()->addError('An error occurred');
        \Drupal::messenger()->addStatus('But then something nice happened');
        $messages = [
            '#type' => 'status_messages',
        ];
        $this->render($messages);
        $this->assertRaw('messages messages--error');
        $this->assertRaw('messages messages--status');
        // Tests display of only one type of messages.
        \Drupal::messenger()->addError('An error occurred');
        $messages = [
            '#type' => 'status_messages',
            '#display' => 'error',
        ];
        $this->render($messages);
        $this->assertRaw('messages messages--error');
    }

}