Add unit tests

[preveyj]

I want to add unit tests to help check my work and assumptions, but I don't know what Lua unit test frameworks exist.

Plus, this being created for CC:Tweaked, I need to look up the CC:Tweaked documentation and see what exists for expected returns and exceptions and such for each call.

[preveyj]

Eventually I'll want to add a Github Actions pipeline to run these tests for me, along with whatever other general checks I might want or need later.