firmware-test: Don't introduce parity errors when nobody is listening

This would cause an error to be flagged, becaue no parity error would be
reported, but when there is nobody on the bus, we can't really expect
them to report a parity error...
firmware-test: Make the sketch work with the IDE

Before, it was only tested using Arduino-Makefile, which doesn't do all
the pre-processing the IDE does. This preprocessing turned out to break
the code, so all code was moved into a .cpp file to prevent it from
being preprocessed like a .ino file. There is still a dummy .ino file,
to make the IDE recognize the directory as a sketch.
firmware-test: Make sure the sketch works on the pinoccio board