WIP: Initial tests for slskmessages

[droserasprout]

First attempt to cover slskmessages module with unit tests and then clean up things a bit

[toofar]

Is the intent of these tests to show that the python3 migration hasn't broken anything or to allow for less guess work in refactoring? Because if it is the former they should be merged to upstream so they can be run against both versions and the latter I think it make sense to wait until after the python3 stuff is merged? To not make the PR even bigger and harder to review.
I'm not saying more tests are bad, just trying to temper enthusiasm for fixing everything at once in case the initial migration gets bogged down in feature creep. In particular I would like to add tests around the networking stuff to make refactoring it to be more efficient and handle more connections easier.