UnitTests #68
Description
We need unit tests, as soon as possible.
GameState.addEntity() was changed to use entityID = entites.size() instead of an seperate variable entityID that is incremented.
This caused an issue in which entities coud have duplicate ids, which could have been avoided easily with unit tests. Note that this issue technically also exists for addPlayer(), however this is not a problem right now.
For example imagine this state:
Entity0
Entity1
Entity2
Now kill off Entity 1.
And then spawn a new one, as a result we get this
Entity0
Entity2
Entity2