Irish Tempest - IMHO you need to be very careful with your terminology here. A 'Test Pilot' is one who has completed a course at a 'Test Pilot School' [like ETPS]. These guys test the aircraft and systems to explore the edge of the flight envelope, before deciding how far the 'average' pilot can go before the situation moves beyond his capabilities [and thus the limits imposed in the aircrew manual/release to service]. I once flew a ME ac that was deliverd by the manufacturer's TP. Next day I flew the post-major Sqn acceptance check, but the TP had to fly a specific part of the flight to finish off his own manufacturer's test because it was to be flown outside the limits imposed on Service pilots.
'Airtest Pilot' is the guy on the Sqn who is experienced and given the job of checking that aircraft and systems behave in the prescribed manner after repair/maintenance. He need not be a Test Pilot or even a QFI to carry out the task.
...Unless John F can come up with a more accurate explanation?