Add tests #591

In the past while I modifying the core code better and cleaner, I have changed stuff quite blindly, without fully knowing the complete image and possible side effects of what I have done, and I ended up spending tens of hours to modify bits of code.

Even top-level code can break very easily without tests (#584 is a perfect example), so going on forward we should prioritize making a tests suite for making sure we don't break anything via possible rewrites (GameDig 6 will be all about rewriting old code).