I have found that there are different types of pilots out there. Few professional pilots I have flown with are "bad", but some good pilots, are not a good choice for "test pilot". Some know it, some don't, it has been my task when flying two crew to determine this. Often, I was wonderfully mentored on the type, and allowed fly what I needed to fly. Some experienced pilots understood what I needed to do to demonstrate compliance, but did not want to participate, and a few simply showed themselves to good pilots who should not fly to the edges of the envelope. A couple of times, I had to return, and very diplomatically ask for a different second pilot. This, mostly, as the second pilot demonstrated an uneasiness with the specified and briefed test point/maneuver. This could be seen in that they tried to "help" on the controls. A number of times, it was necessary, and wise that I be checked out on a new type, before test flying it. For some very basic "check" type test flights, I could really just do it during an hour long checkout flight, and suggested that, to save the second flight. I have a number of recollections of a check pilot saying to me, no, you're the test pilot, I would rather not do that stuff, you're checked out, take me back, and do it on your own. Okay, I never want someone aboard, who does not want to be there!
I know a few several hundred hour, fairly new pilots, who I would send, with a good briefing, to do a number of tests. And, I know some multi thousand hour commercial pilots, whom I would not allow to fly more than a 30 degree bank, or within 10 knots of stall speed. It's only partly an experience thing, its also a hands and feet, and relaxed/fully aware of what's going on around you thing. The result is that without flying with someone, I can't say if they are suited to test flying or not, I've got to fly with them. But, I do know that the candidate pilot must understand the plane's capabilities and limitations, and understand the sensations of being there in flight.