As one who has taught at three different test pilot schools, worked for a manufacturer (briefly) and also for a civil certification authority, and been on the membership committee for Society of Experimental Test Pilots, perhaps I can add a few words.
The whole area of flight testing is so vast, and has so many components to it that it would be impossible to set up criteria that would satisfy every one.
More than 50% of the flight testing that is being done now revolves around avionics and displays, for example. How do you set standards for what is going to be taught for this subject? Do you cover this at the expense of performance?
It has been modestly predicted that even a very cursory look at all the areas of certification would take a minimum of 18 months, if not two years at a flight test school. No-one could afford to have an experienced pilot on their staff away for that long, nor afford the cost of the course.
Like Genghis and Rick, I have seen pilots and engineers who are not graduates of formal training who run circles around those who have attended Test Pilot Schools, just as I have seen those who are self-taught who shouldn't be allowed to drive a car without adult supervision.
I wish there was some way to give some structure and form to the whole process, but it would be nearly impossible given the number of different interests involved.