To me, no test drive=not interested, and I would be quite annoyed if someone waited until I came to look at the vehicle to mention that. But, what I do think is perfectly fine, and what I have done when selling vehicles, is to drive it for a little bit with the prospective buyer as a passenger, then switch places and let them drive it for a while. I also won't test drive a vehicle without the owner in the car with me, I don't want to be blamed for breaking something that actually wasn't my fault.