I like those Coffee Breaks. There are just a bit of amusement between the real tests. You only have to complete it to get Bronze, there is no Minimum time required.
They don't count for anything, they aren't a test and they don't make up part of the 80 tests in GT4 - there are 80 License Tests plus 5 Coffee Breaks.
I've got the 5 of them in Gold and, as Famine already said, there is no prize car. No prize at all.