Add unit tests to the project

[Moneyl]

Unit tests should be added to the c++ code of the project to try and make the code more stable and to notice breakage early. Ideally they would automatically be run by appveyor/azure pipelines/github actions each time a new commit is made.

There isn't any code coverage goal with this, just that the important areas of the code should have meaningful tests to make sure they haven't broke. We could probably also add tests to the lua code, but it'll need a separate testing tool/framework.