Fatal error: Uncaught Error: Class "Drupal\Tests\Component\Serialization\YamlTestBase" not found in /home/www/drupal8/test/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/tests/Drupal/Tests/Component/Serialization/YamlSymfonyTest.php on line 15