Fix for rc_test_motors doesn't run all motors

Created by: PeteLawler

From the help output, not supplying a motor number should run all motors. However, the test previously insisted on a motor number being supplied even though the default was to run all numbers. From reading the library, I think this should work but I'm not 100% sure. Don't have machine at hand to test on right at the moment. And my C is a little ancient, too....