Cleanup tests

[nhooyr]

Instead of helpers to send and construct frames, the tests should be described as a sequence of sent and received frames.

Now that would be neat and reduce a lot of duplication and verbosity.

[nhooyr]

See #129

[nhooyr]

the TestConn tests should also be changed to not require server/client, separation, we should test both sides.

[nhooyr]

Also want to up coverage to at least 95% or higher.

[nhooyr]

This doesn't really deserve an issue, the tests are fine as is. Will improve them as I find the time.