Increase test coverage

[enobufs]

We've seen a few bugs in pion/turn which could have been easily detected earlier if there were sufficient test coverage. With this ticket, let's aim for 80%+ coverage at least for ./internal/client package.

[songjiayang]

I hope that I can contribute a little strength 🆙

[Sean-Der]

I just added +4% with Client+Server tests for stale nonces!

@songjiayang @enobufs I will start a list of easy wins people can write tests for :) this is also a good chance to refactor/cleanup anything we run into.

[Sean-Der]

The client code has quite a few refresh related things that aren't tested.

We should allow the server to have very short expiration times, and then force things to be run conn.go

[Sean-Der]

The server has quite a few easy tests as well, we can send invalid packets in and confirm that an error gets returned/things don't crash turn.go should be easy to test