[ActiveJob] TestCase

@dhh @rafaelfranca @cristianbica 
Late to the party, but this was referenced in my PR #16992. Looks like this got merged with something that looks like it's not global, but really it is, since the test helper sets AJ::Base.queue_adapter (globally) during setup, and reads from the (global) AJ::Base.queue_adapter on every assertion. Thus, concurrent tests all still stomp on each other. :crying_cat_face: