Fatal error: Uncaught Error: Class "PHPUnit\Framework\TestCase" not found in /home/www/drupal8/test/core/tests/Drupal/Tests/Component/Render/PlainTextOutputTest.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/tests/Drupal/Tests/Component/Render/PlainTextOutputTest.php on line 14