Unit testing

[MattSturgeon]

Minecraft is a bit of a pain for unit tests, you can't easily pluck out a single class without first initializing the rest of the game.

That said, starting to add unit tests will make it easier to make changes more confidently, and make it easier to update to new versions/maintain multiple versions.

Long term, we'll end up improving the code base by refactoring classes to be more "testable", but initially we can start by just getting JUnit setup and testing the low hanging fruit like FreecamPosition.

I'm not sure what options are available to help setup tests and mock Minecraft classes. I've seen this, but it only targets older versions of the game.