KeyTestTrait not found

The KeyTestTrait class does not appear to actually be needed in the code. Commenting it out silences the error. I'll make a fork to do that for now.

Is this needed for the integration with the Simple Oauth module if oauth authentication is wanted?

This is also happening in the 4.0.x version

Folks -- commenting out bits that don't work is a HACK, it's not something you post to a MR!!!

Key module is required for the tests -- it's in the module's composer.json's 'require-dev' section.

Arguably, we shouldn't be using it, at least not for general authentication tests -- we support storing credentials in the database so we should use that, to keep the tests simple and covering the least possible surface. If someone feels like working on that, then please do.
We probably would still need at least one integration test with Key module though.

Thank you for the background, @joachim. Given that, it sounds like "works as designed" is a more appropriate status for this ticket.