Add Unit Tests

[mpriscella]

Unit tests should be added to avoid regressions when changing logic. It seems that the most mature unit testing framework is zunit, which is based off BATS.

It's not feasible to get 100% code coverage in one go, so the scope of this issue should be limited to just a couple proof of concept tests.

• Write proof of concept zunit tests
• VSCode task definition to run the zunit tests
• GitHub action that runs zunit tests on PR creation / modification and reports failures