Include Unit Testing examples

[tdcox]

Ideally, educational examples should include some simple unit tests to demonstrate the requisite patterns for testing in a new environment.

This is particularly important for embedded systems since cross-compilation introduces significant testing challenges. There are several scenarios that should be demonstrated:

1. Testing Swift 'business logic' on the host machine - compiling for the host and executing tests locally as part of the build.
2. Testing on target hardware - cross-compiling and then launching a test suite on target hardware and collecting results.
3. Testing on emulated hardware - cross-compiling and then launching a test suite on a simulator / emulator running on the host.

Consideration should be given to managing external dependencies like FreeRTOS libraries and physical hardware that may have limited emulation support.