Yes, tap water is very bad for a water loop, and yes, it will cause damage. However, a few hours of leak testing will not produce any undesirable effects. Sure, on a microscopic level it might induce electrolysis, but are you going to notice a difference in temps or see discoloration in your block....no.
I have been reading Procooling for years now, and I understand what you are saying. However, I have done this many times, once again, for only a few hours at a time, and I have never seen any negligible difference in either performance or color of the copper.
Edit: I say this ONLY if you are using an entirely copper/brass loop. If you have aluminum in it, then NEVER use tap water.