Write tests

[rukai]

Gameplay should not be tested. (at least at this point)

A simple functional test could be:

1. the game can be booted headless cargo run --no-default-features -- -p2
2. wait 10 seconds
3. frame1 = pf current_frame:get
4. wait 1 second
5. frame2 pf current_frame:get
6. assert frame2 > frame1

Also the infrastructure such as package downloading/saving/reloading would definitely benefit from some unittests.