Ok, I found a way to test-drive a car without losing your precious money.
First turn off autosave. (Yes, so you're stuck with the restricted one)
Buy a car (new or used, it doesnt matter), but do not get in. If the game asks if you want use the car, select the option NO. You see the words processing. This is not the same as SAVING. The game will process WITHOUT saving. But the car is still in your garage. 💡
Next thing, you exit GT Mode. The game still doesn't save.

Then go to the arcade mode, select one of the options (for example time trial) then go to the manufacturer and there's your car. You can drive it on the tracks that you have unlocked already. If you don't like the car, you just reset the game and the next time it loads the situation before you bought the car, so no loss of money.
The only thing is you can only test-drive the car in it's stock form, because if you want to tune it, you still have to get into the car and there the autosave kicks in.