It is my understanding that calculations for maximum permitted speeds on very high speed test runs took into account horizontal and vertical curvature and pressure wave effects (important at over bridges and through tunnels) but that a safety margin was still included (probably no more than 5%). The drivers would have been given the resulting maximum permitted speed graph. The speeds shown on it would probably have borne little relationship to the planned service speeds for the line. I obviously don't know about this run, but in the past very high speed test runs have been authorised to run at speeds in excess of 500 km/h where the line was suitable - and such speeds have been attained.
My first thought when seeing a range of photos of the accident was that the train was running too fast for the vertical curvature and literally took off. Pepy's statement doesn't seem to totally rule that out although I have only seen an English translation at the moment, he speaks excellent English but I can't imagine that the actual statement was in anything but French.
French drivers were, and are, trained to drive to the timetable, being never more than 2 minutes early or late, but, yes, they were expected to observe speed limits strictly, and this would be even more true of TGV drivers as driving at a speed above the steady-displayed limit would cause the over-ride to cut in and stop the train.