Review how to call the correct base Class. Review Drupal core's file pathing to kernel tests.
In core, the PSR-4 'tree' starts under core/lib/. https://www.drupal.org/docs/develop/coding-standards/object-oriented-code
The ControllerBase is a utility base class for thin controllers. This is used to load pages/controllers. Drupal 8 uses the Symfony HTTP kernel system to request and produce a requested object - a response object to send back to the client. Read more on Drupal.org's Underlying functionality of the routing system in Drupal 8