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