Fatal error: Uncaught Error: Class "Drupal\Tests\UnitTestCase" not found in /home/www/drupal8/test/core/modules/book/tests/src/Unit/BookManagerTest.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/book/tests/src/Unit/BookManagerTest.php on line 12