Lots of people (most maybe?) can drive someone else's car under their own insurance but there's a big BUT - it's only third party. Thus if they were to crash into another car while test driving yours (ie the test driver's fault), then that other car (the third party) would be repaired under the test driver's policy - but your car wouldn't. If, however, someone else were to hit you (ie their fault), then their insurance would pay to repair your car.