Crab - the import point you seem oblivious to is that flight in question wasn't a testing flight it was a training flight. Makes me wonder if you fully grasp the difference?
A well conducted training flight with a fairly broad agenda would, as we've already mentioned, start the briefing room. One would get the student to talk through the various excercises that might be contemplated and it would rapidly become obvious when a weak spot was reached, either because he was unable to describe, e.g., how to deal with jammed pedals, or because he said "yes, I've alway found that really difficult". Of course this requires the instructor to be in receive mode, something some struggle with as they are stuck in transmit mode - often transmitting to ensure the student is quite clear just how much more they know / how clever they are.
A refresher brief then ensues and you then go out to practice it in flight. A productive training session!
If you just pick an emergency at random in flight, there are only two possible outcomes, one is that it all goes according to plan, in which case it was a waste of flight time, or it doesn't go to plan in which case the PPL merely learns that he can't do something. Also a complete waste of time.
Testing is of course a different matter but it is not what was being talked about. I really do think some instructors struggle with the fundamental but vitally important difference.