I have an engineer instructor friend who used to make a living training engineers in simulators to qualify them to do this sort of thing. I think they used the same full-flight sims as the aircrew but perhaps they used fixed-base training devices; I never asked closely but I know that it certainly isn't just a matter of "Hop in there and give that thing a full-power run-up, would you?"
Perhaps it was the first time he had done this without other, more senior, Airbus people there to keep an eye on things. Probably the last time, too!