Add unit testing

[pedzed]

I would love to write unit tests, but I couldn't get it working when trying to manually set up my environment. I decided to use Easy C++, but to my amazing unit testing is not added at all.

Feature request: add a way to write and run unit tests. I don't really mind any testing library, but I think a choice should be made between Catch2 and Google Test. Or add both as a choice, if that's possible.