Move to pytest, break up tests

[Ian Cooper]

Although unittest is a reliable workhorse, we should probably move to pytest.

When we do that it would be good to separate core tests from tests that run against specific transports. This has worked out well for Brighter, as it allows you to choose what tests to run during development, often speeding up the flow by not requiring that middleware is running at the time, and encouraging a clean separation between the core and middleware