If it's not asked, how is it lying or feel guilty about it? Would the accident been prevented IF he was in the car with you? If not, let it be.
The dealers insurance coverage only applies IF a dealer employee was in the car with you at the time of the roadtest. Thats why he said "say that I was in the car with you". Loop hole for the insurance company to tell the dealer to pound sand. You could sue the dealer and wait to let it all get worked out over a long ass period of time or just keep your mouth shut unless asked directly if the salesman was in the car at the time of the collision. There insurance company will go after the other driver so why make more waves?
I don't even think it matters about the dealers ins. When is the last time you saw a rear-end accident where the one who got rear-ended was found at fault. FFS