Fatal error: Uncaught Error: Class "Drupal\KernelTests\KernelTestBase" not found in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php:21
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php on line 21