run-tests.sh error: Test class not found
The solution ended up being:

Make sure the test file is named as the test class, in this case RSVPListServiceTest.php.
Make sure my unit tests are placed in the correct directory structure, in my case my RSVPListServiceTest.php file had to be in modules/custom/rsvplist/tests/src/Unit/EnablerServiceTest.php (as opposed to unit with a lowercase u)

The ERROR: No valid tests were specified. message ended up being a bit of a red herring, and I've submitted a patch to drupal to make this message clearer: https://www.drupal.org/project/drupal/issues/3170651