Are transport category aircraft stalled at high altitudes in certification testing?

[actus reus]

'Also the stall itself (maximum lift, loss of controllability) occurs "earlier", at a lower angle of attack than at low altitude.'

So true.

Many pilots that I have asked still think that in the cruise at altitude the aircraft will stall at 16 degrees AoA.

The 'buffet' clue with a semi-supercritical wing; another thing again.