Everyone in this thread seems to be too paranoid. It's not common that a car gets stolen during a test drive and it's not common that someone gets robbed during test drives. If the car gets wrecked during the test drive then the owners insurance is responsible, but the likelihood of the car being wrecked during a short test drive is very slim. And yes dealers do let you take cars home for the weekend, GM used to advertise the 48 hour test drive.
Yea, there is a lot of paranoia but this kind of shit isn't new, a friend of mine in the '70's had a mint Honda CB750 up for sale, a guy read it in a local paper and asked to come by and have a look-see, he arrives in a nice Mustang and after some discussion asks for permission to take a short test-drive, friend says OK considering he was leaving the Mustang there which was worth a lot more than a Honda 750, well the guy never returned, when my friend went over to the Mustang it had been hot-wired, it was reported stolen the day before. '60's and '70's vintage cars were very simple to hot-wire, so much so that most who valued their cars had a "kill-switch" installed to help out..