Write unit tests

[mtangoo]

Use Catch2 located here: https://github.com/catchorg/Catch2

[Blake-Madden]

@mtangoo You can assign this to me if you would like, I'm working on this now. I have as a CMake project that fetches wxWidgets and Catch2 at runtime (so that they don't need to be included as submodules).

[mtangoo]

Thanks for that. I have assigned it to you!

[mtangoo]

It will make PR review and merging a lot easier