Hello mate
I totally understand your worries about the mechanic's ham fisted approach to his "testing" the engine but although the other guys have given you perfectly good advice I really wouldn't worry about it too much if you can.
Yes, he should have not done it and i'd feel the same as you but a one off like this will in all probability make no difference to your engine at all. I dont know the mileage of your motor but chances are its done a few thousand by now and every engine "finds" its own tolerances in the end...in other words it not going to be a tight new engine and that'll help no end.
The other thing of course is from the sounds of it the motor was being revved with the car in neutral and no load at all on the engine. This will certainly help, a free spinning engine is under much less stress than one being driven under load. From what I can gather, Fiat have made a peach of an engine and yes, its definitely bad form to rev one thats cold but I would bet you that Fiat themselves have tested the engine (as a type, not yours) by being far far rougher than any normal human being would ever do.
Without being rotten to Italians I just know that they all drive like nutters and Fiat would want to guarantee that their engines will not fail in the hands of a native
So...try not to worry mate, a one off like this probably wont make any difference at all. If you drove it like that every day for years then yes, it probably would start to complain but otherwise, you'll make yourself more poorly by worrying than the engine will ever get
Take it easy then bud