Well, I wouldn't want to take a dmv test in a truck either!
But, I'm sorry, even if your parents are fully qualified to rent a car (valid drivers license, credit card, over 25), the car rental companies require that all additional drivers must meet the same requirements as a renter, and they have to complete and sign an additional driver form at the rental counter.
Even if you were old enough to be an authorized additional driver and had the required credit identification to show at the rental counter, because you don't have a full, valid drivers license, you would not qualify as an additional driver, for any rental car company and cannot drive a rental car for drivers test -- or at all.
I would suggest contacting local driver training or driver education companies in your area (try doing a search on yellow pages.com)... often these companies have programs where if you purchase training time, you're able to use one of their training cars for your drivers license test.
Pages from this site, relating to this topic:
Car Rental Policies Index
Drivers License Requirements