Improve test coverage

[DanielOaks]

Getting the test coverage way up would be good. Getting a bit of green in that icon and all! One of the problems is that we link things quite a bit, for instance the features module has to take a ServerConnection to do its job. Stuff like that is stuff we should replace with better methods and linking. Maybe take inspiration from girc-go, if there's anything in there we can use.

There's also our irctest module here, but that's more to test our compliance with the IRC protocol in general rather than what proper test coverage does.