Does gamestop test used systems?

Yes, they (are supposed to) test incoming used systems the moment they are traded in. If the system is defunct at time of trade, it is sent out to their warehouse to be refurbished. The reason they test them is to off-set the cost of the refurbishing - so if they take in a broken system, they deduct a little off of the trade itself.

The gaming being in it may have actually BEEN from them testing it. Of course, when they test it they need to pop in a game to make sure it works... so there's a chance you have their copy of the game...

They don't test the actual games, though, as it would be far to time consuming.

nope