Excellent response Genghis the Engineer - one that accurately represents flight test as I have known it.
Accreditation is an oft discussed subject in and out of flight test. I have heard arguments that since the Empire Test Pilots school is recognized as the oldest test pilot school it should be the one to decide what the standards should be for accreditation and what experience and knowledge would be necessary for accreditation on a worldwide basis. Setting aside political considerations, the other test pilots schools can't always agree on which TPS organization has the highest standards and should be entrusted with setting the standards.
There are some who have discussed a civil test pilots license or certificate in the French fashion.
Responsing to an earlier post by Affirmation; in the US, a Flight test pilot is defined in Title 14 CFR Aeronautics and Space in part 21 Certifications procedures for products and parts:
§ 21.37 Flight test pilot.
Each applicant for a normal, utility, acrobatic, commuter, or transport category aircraft type certificate must provide a person holding an appropriate pilot certificate to make the flight tests required by this part.
[Doc. No. 5085, 29 FR 14564, Oct. 24, 1964, as amended by Amdt. 21–59, 52 FR 1835, Jan. 15, 1987]