I don't know of one that's used for testing. YPN isn't CPM advertising though labnol. It's CPC. You're not going to cost anybody anything by simply showing the ads. I do understand what you're trying to do and I think it's a good idea, if for no other reason than to prevent impressions from being artifically inflated. Again though, impressions aren't going to cost your advertisers. Only actual clicks.
What I do is just stick something in there in place of the ads. An empty table cell maybe? Something with the same width and height dimensions that correspond to your ads width and height. If I'm doing extensive testing on a page (lots of reloads) I always do this. Other people might have better ideas about this but this is what I do.
But if I use some existing ad partner value and accidently click on the ad
Well, how many times is that going to happen? I was an adsense publisher for 2 years and only accidently clicked twice.. both times were when I thought I was right clicking to get the URL and mistakenly left clicked. And no, I didn't immediately email adsense and fess up.. I'm sure they get enough of that as it is...