Unit Testing

[atticusrussell]

Establish a framework for unit testing and implement some examples:

• Use PlatformIO testing with the Unity framework

• Add detailed comments to files in firmware/lib/motor and firmware/lib/pid

• Create native unit tests for PID

• Create embedded unit tests for motors (2 of 4 types so far - generic ones)

• Add running native (desktop) unit tests to GitHub Actions CI

• Describe Unit Testing in README

Other unit tests should still be added in future PRs, but this should serve as good example for developers writing unit tests for features.

[atticusrussell]

@grassjelly thoughts?