No VNA I don't think you will find they do! Sometimes the student will fail through no fault of the FI, just pure nerves. As mentioned above you will get ones that demand to do the test because they think they are ready when they are not and then fail it. I had one fail because he set off on test with nothing in the aircraft to enable him to identify the VOR's and had bought himself a new plotter the day before the test and didn't know how to use it. Examiner had to intervene to prevent controlled airspace penetration as a result of the latter.
A dummy skill test is required in my opinion. One of the problems for the student is a properly conducted PPL skill test will last comfortably over two hours. The student will never have flown a detail of that length of time and will thus find the test very tiring. So in my opinion you have got to put them through a dummy test, and the one I do will last about the two hour mark.
Any weak areas will require further revision before I put them up for test.
Being of not that greater experience myself I always talk to the Examiner afterwards as to how things went in case there a things that I have missed or not taught particularly well.