Testing: unify http client mock

[richardm-stripe]

Summary

It adds a lot of complexity / opportunity to get things wrong in our testing framework that you always have to specify http_client_mock_streaming vs http_client_mock (and in the beta branch, it balloons and you have http_client_mock_async and http_client_mock_streaming_async). This PR changes HTTPClientMock, instead of constructing it with is_streaming and this determining which self.func was set, we instead have self.funcs and combine the mocking functionality to operate over multiple mocked functions.

[anniel-stripe]

Thanks so much for doing this!!