Refactor integration test

[richardm-stripe]

tests/test_integration.py is a test file that spins up an HTTP server running in a separate thread and is useful for tests that need to be written at the HTTP level, but I ran into some difficulties when attempting to add a new test here. This PR refactors the test helper a bit to make it more usable:

• Instead of producing assertion errors inside the HTTP Server thread and doing a weird conversion into HTTP 400s, instead the HTTP server thread sends the request objects back to the test thread via a Queue and you can assert on the received requests inside the test.
• There is a sensible default implementation that returns 200 with empty JSON.
• Now, to define a test HTTP server, you just subclass TestServer and (optionally) implement do_request(n: int) to return [status_code, headers, body]`

[pakrym-stripe]

I'm not sure why these tests need an actual HTTP server and can't be done on top of HTTP client mock.

[richardm-stripe]

I'm not sure why these tests need an actual HTTP server and can't be done on top of HTTP client mock.

Some of the logic being tested here is logic that is implemented in http_client.py itself.