I don't care to be the one who defends a dealership; but, if the owner sincerely trusts his employees, he may believe that when the mechanic got the car up on a lift, he may have seen the damage and said to himself, "I better document that since the service advisor didn't write it down."' The owner may have been burned previously by persons who have brought cars in and then claimed damage was done by the dealership, when it was not. It's hard to believe, but, there are people like that. Another scenario that the dealership owner might picture is that the owner of the car may not know that his 17 year old son and his buddy snuck the car out the night before and had a little accident that Dad and the service advisor didn't notice.
However, since the service advisor didn't document the damage when the car was signed in, I believe that the burden of proof is on them, and they should pay to fix it.
Like plumcrazy said, you should get them to send you the photo file if it exist, or, eventually have your lawyer get it. If they send the original photo file, the time that the photo was taken may be tagged in the file (at least on my camera, the photo file includes the date, time, & camera model. That relies on them having set the time in their camera, though. If the photo was taken after the car was test driven, then that is great evidence for you. If it was taken 15 minutes after you dropped it off, then it doesn't help as much. Since you say that both wheels were damaged, it most probably was damaged during the test drive rather than when putting it on a lift.
Good Luck !