Thanks for your review.

1. Based on what I found on the internet unit tests are to check on a test case if the result of some function is correct. What would it be good for in this project? I really don't see any good use of it.
2. Yes that would reduce the overall number of calculations, but it wouldn't really say it simplifies the logics of move() where the main problem are all those checks when the head changes to another area of the cube. But thanks anyway. I'll implement it the next time I work on Snake3D.
3. Yes. That's what I already started doing by making size refer to that one constant in main.