It would come down to the reliability of the protective function, whether it can be overridden by the pilots and whether the manufacturer wishes to have dispatch allowed with that function inoperative.
For autothrottle, every manufacturer would want the MMEL to allow dispatch with autothrottle inoperative, with no requirement to reduce the maximum allowed speed, so I would be very surprised if they would propose to take credit for autothrottle to reduce thrust during this test.
FBW systems often include overspeed protection functions that cannot be overridden by the pilots, and these systems are operative during these flight tests. The pilot must push the control further and further ahead as the aircraft accelerates during the dive, and eventually ends up with full forward control and the airspeed or mach stabilized at the limit set by the automatic high speed protection.