motor testing backend for UAVCAN ESCs

The motor testing interface offers easy drive testing via command line utility ('motor_test') or via GCS/MAVLink (AFAIK currently only supported with APM). This PR adds the necessary backend implementation for UAVCAN ESCs. To allow for testing of single drives without idle-spinning all motors, it was necessary to implement support for arming single drives.

Code style: else please on the same line