Fatal error: Uncaught Error: Class "Drupal\Tests\UnitTestCase" not found in /home/www/drupal8/test/core/modules/serialization/tests/src/Unit/Encoder/XmlEncoderTest.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/serialization/tests/src/Unit/Encoder/XmlEncoderTest.php on line 15