San Francisco

well turns out you cannot just rent a car to do the test. DMV here does not tell you they need a letter from rental agency to do the test. So I didn't have that, and they turned me away. Balls, now I asked the rental agency, they said no. Wtf ? I can rent and drive it just fine (and legally) but you won't let me do my US test in it ? What the hells the difference ?
So, now I'm having to track someone down to borrow a car

That's really strange. All I needed to sit my CA driving test in Los Angeles (nearly 2 years ago) was make sure that my rental car had California plates on it (worth noting as plenty of rental stock here has Nevada or Arizona plates) and have proof that I was insured on that vehicle - ie, my Rental Agreement from Hertz. I just showed up with that and it was fine.
I also didn't have to surrender my UK Licence (some people will tell you that you must, but do whatever you can to hang onto it).