Fatal error: Uncaught Error: Class "Drupal\simpletest\WebTestBase" not found in /home/www/drupal8/test/core/modules/responsive_image/src/Tests/ResponsiveImageFieldUiTest.php:14
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/responsive_image/src/Tests/ResponsiveImageFieldUiTest.php on line 14