Proper quattro with a torsen diff is absolutely fine on 2wd brake rollers as the torsen can easily accomodate the small amount of rotation. Haldex though, not so much, as the rear diff clutch would be locked up so it would either destroy the clutch or the diff. That would need the wheels rotating in opposite directions as Adrian described. I've never seen that happen though.
Most MoT testers I know just do a road test, some with a special weight-gyroscope sort of thing to measure the braking force, - my current tester particularly as he had to pay for a new diff on a TT quattro when they first came out!
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