Create auth test suite that is run for all auth subclasses

#197 was able to occur because we didn't test that `JWTAuth.refresh()` followed the same rules as `OAuth2.refresh()`. There is an `OAuth2` test case for the scenario that triggered #197, but not a `JWTAuth` test case for the same scenario.

We should have a common test suite that can be used to test the common interface of all auth subclass. And then make sure we run that suite for all of the subclasses defined in this project.