Provide helpers to mock critical infrastructure components for testing #128
Description
It would be nice if we had some mock adapters, aiohttp servers, and other authorization-related classes to facilitate unit and partial integration testing. We already do this in the testing code, but it's a bit disorganized and not at all consolidated. This makes increasing our test coverage for existing and new features take longer. Also, providing mock/testing objects could allow us to expose these to users for their own tests.