Add Java tests

[CosimoTiger]

[CosimoTiger]

Commit 04108f3 adds the first JUnit tests for the non-Bukkit Cooldowns code, but I'm not sure how I'll make the tests for the Bukkit code even though the mocking libraries for that already exist.

[CosimoTiger]

Commit e38e135 adds some basic unit tests for chest menus and the Slot utility.