I've sent a few things back and they have tested them (sent back a gpu and they told me what they had to underclock it to in order to get it to work, sent back a case and they had ended up replacing the pcb on the front panel as it turned out to be the problem instead of a bad cord) so i'm pretty sure they at least do basic testing
if under warranty, the worst they do is make you pay for shipping from my experience. I didnt have to pay restocking on the gpu i got that was defective