mixer multirotor: add unit-tests

To run:
cd src/lib/mixer
make tests

This will validate the C++ implementation by taking the python
implementation as ground-truth. It runs through various actuator control
command values for all airmode variations and several mixer types.

The python script also allows to prototype new mixer algorithms.

It is not integrated into the existing build system, because it's easier
to use that way, with less dependencies, and faster testing workflow.
It could however be a bit more integrated.

Reference: https://github.com/Auterion/Flight_Control_Prototyping_Scripts/tree/master/control_allocation