Create auth test suite that is run for all auth subclasses

[jmoldow]

#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.