Improve tests and testability

[dazuma]

Many of our tests are currently "integration" tests in that they test the total behavior of the game for a certain sequence of events (e.g. kill a tank and pick up its power-up).

We should probably make more unit tests that test functions, or single actors (i.e. send a message, receive response messages)

[BitBrit]

To this end I wrote one more unit test for power ups. ;-)