Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/core/modules/block_content/src/Tests/BlockContentTestBase.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/block_content/src/Tests/BlockContentTestBase.php on line 15