That can happen. And those might have been manufacturer problems which may or may not have been visible. If it didn't happen on the very first dive (you said they were "near new") then the test wouldn't have helped. It can always happen but every one who takes a camera down has to accept that this is simply part of the risk on each and every dive.
However, just coz you test it empty doesn't mean that problems like that will show up in the test.
I've been taking uw photo and video stuff underwater for a lot of years - 7 video housings and 10+ film/digital housings. Never tested any of them, never had a problem.
There are also differences between manufacturers so do your homework and talk to people who have used the rig you are thinking of - some have better reputations than others. In the end, it's always possible to get a dud...no matter how much you spend.
If you are going on holiday and are concerned, it does pay to check out the possibility for replacing a camera or hiring one - just in case. Then check with your insurance company to see if you can simply do what you need to do right away and get reimbursed...some are very quick after the initial phone call so you can get back in the water again.
Investing in a backup camera can be an option, too. Get one that fits the same housing...most housings for digital cameras aren't electronic, just push buttons, so drying it out really well after the F-word and sticking in the second camera gets you back in action. I always carry a second camera (may or may not fit my housing, I live on the edge, you know
) for land stuff as I hate to drag the housed camera out of the housing for topside stuff and lugging it in the case is too much effort - and in hot climes not such a good idea.
Could it happen? Heck, yes! Am I going to waste a dive on the possibility? Nope
Am I insured? You betcha
Do I have a backup system? Most usually
Remember - shopping is FUN! lol
YMMV but always remember - this is meant to be fun stuff, so don't worry too much about it...just be careful and thorough.