Fatal error: Uncaught Error: Class "Drupal\Tests\BrowserTestBase" not found in /home/www/drupal8/test/modules/migrate_tools/tests/src/Functional/SourceCsvFormTest.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/migrate_tools/tests/src/Functional/SourceCsvFormTest.php on line 18