Add test for Topics msg

[quietlychris]

[quietlychris]

Work has been started for this in the topics branch; right now, the TCP test is working, UDP is failing, and QUIC is not yet implemented.

This is also demonstrating a need to re-configure the test methods; unfortunately, Rust doesn't allow the passing on non-lifetime generic parameters for tests, but using the type N = Tcp/Udp/Quic pattern, we should be more easily able to replicate every test across the transport protocols to make writing tests/extending coverage a bit more clear.