Fatal error: Uncaught Error: Class "Drupal\Tests\UnitTestCase" not found in /home/www/drupal8/test/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/tests/Drupal/Tests/Core/Render/Element/MachineNameTest.php on line 18