Add unit tests

[TDHolmes]

We've added CI to ensure it builds & we don't get clippy regressions, but this doesn't prevent runtime issues

[TDHolmes]

It's unclear to me how to run tests on an actual pi. Does GitHub support docker easily?

[TDHolmes]

Looks like we would need to setup our own GitHub Actions runner to enable this, or have the C API mocked out like #12 was trying to do. In the meantime, the existing unit tests do work, as long as you specify --test-threads 1 in your invocation of cargo test.

[BroderickCarlin]

@TDHolmes Github does support Docker easily as a part of workflows if you believe that is beneficial

[TDHolmes]

surprisingly, the C++ library compiles in normal ubuntu, so I think we could do jelly bean GitHub actions compilation. However, the actual runtime behavior might be weird :p Lets wait and see for #1 since before then, linking properly will be... tricky.

[TDHolmes]

I tried to get this going with #27 without success

[TDHolmes]

Tests using the stub functions would work though, however most of the library relies on the FFI so not sure if that's beneficial