Unit tests for mixer

This patch set contributes two things:

  * It fixes the pwm limit library call, whose state machine was inconsistent, resulting in memory (garbage) values being used for the first 0.5 seconds during arming. The state machine needs to be reviewed again now, but it should do what the design originally intended now.

  * Added a tight unit test for the ramp and the pass-through mixer, ensuring the ramp behaves as intended

Missing: A unit test checking arming / disarming cycles and the same thing tested in real-life. Please review the changes, I will add the test in the meantime.