Well it would seem that none of them tried to access the flight deck, so they wouldn't have known whether or not it was actually locked, which I'd imagine would have been one of their main objectives if it were a "dry run". Since the only other realistic way of bringing the aircraft down would probably have to involve a bomb of some kind, why all the nonsense? This doesn't quite all add up to me.
I'm not saying it wasn't part of some kind of terror prep, but it seems more likely that there is an innocent explanation for their strange behaviour. Perhaps the guy simply needed the toilet in a hurry? Perhaps the others thought it was okay to get out of their seats and whatnot? Let's not jump to conclusions.