Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/modules/contrib/htmlmail/src/Tests/HtmlMailTest.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/htmlmail/src/Tests/HtmlMailTest.php on line 12