Arduino sketch unit tests for SPI chip-select multiplexing

[ethanjli]

A suite of Arduino sketches should be written to exercise the SPI chip-select multiplexing functionalities. We can start with a simple blink sketch with only PP-T41. We should also write some tests for BP-Jmp, once that arrives. Tests for functionality of other planes might be out of scope of this repo, if they are complex (e.g. if they require writing nontrivial drivers); tests in this repo should be limited to the ODSv1.0.0 stacking architecture.