all you need to pressure test a tank is a gadgue, a pressure washer ( even a cheep one ), couple of hdralic valves and some hydralic hose. just make sure the tank is FULL of water, then just bung the last outlet and connect up the pressure washer.
pressure washer > restrictor > ball valve > T off to pressure gadgue , T off to hose to tank > valve ( to open air to vent down )
tanks whist they dont normaly pop its getting more common. i've got a tank from 50 somthing thats 1/4 thick and seems to have no intrest whatsoever in rusting. but mates had a 4 yr old 50ltr tank go pop, damage wise it chould have done a lot more or a lot less but it knocked a couple of blocks out the wall.
tyers are the ones to watch especaly bulged ones if it looks like it might go i scuff them round till they do now, otherwise 1st job is valve out. mate wrote his van off with a supersingel, chucked a bulged on in the back and chouldnt be arsed getting it out that evening somewhen overnight it went pop bent the doors out and bulged the whole back end