Don't think of it as "used"..
Think of it more as broken in. Most of the components have the settling period. Cables stretch, brake pads need to be seated, etc. With a test bike, all of this has already been done. If you're worried about how hard the bike was ridden by previous "testers", then you should be looking at a different bike or a different company. These are mountain bikes and are designed to be used as such! Even crabon frames these days are strong enough to take shots from 5 pound sledgehammers and still keep rollin'. Having worked in a shop, I can verify that just about every bike on the floor has been tested by the guys that work there, customers, friends who drop by, etc. You should think of it more like the initial shakedown period is over, if something is very wrong it will rear it's head and you can get it dealt with under warranty sooner.