Gimbal: add angular rate test and fix deg/rad input

[julianoes]

This adds a test to test angular rate input, not just angles.

Also, this fixes MAVLink angular rate input which is in degrees but needs conversion to radians. At least this second part should probably be backported to v1.15.

Also, make sure to forward gimbal traffic in SITL as it is required for proper discovery of a MAVLink gimbal.

Tested in SITL using typhoon_h480 model