Fatal error: Uncaught Error: Class "Drupal\Tests\UnitTestCase" not found in /home/www/drupal8/test/core/tests/Drupal/Tests/Core/Render/Element/RenderElementTest.php:17
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/tests/Drupal/Tests/Core/Render/Element/RenderElementTest.php on line 17