Knowing nothing about full power ground engine runs, (I only use full power to get airborne on days when flex power is not sufficient), I am surprised that aircraft being tested statically at full power are not chained or otherwise fixed to hard points in the engine run-up bay.
Relying on loose chocks and brakes and a human’s reactions to chop the power seems a bit dodgy to be honest.
I would have thought that at the very least, some sort of main wheel sized chocks that bolt to the ground would have been used.