Can we have certain knowledge whether or not they're conscious? - Unfortunately no. We can't compare what we cannot measure, and we haven't found any way to measure consciousness directly.

When AI passes all possible tests that could distinguish it from a rat, the question becomes whether or not consciousness is necessary for all those rat-like capabilities we tested for. And if not, then why rats have consciousness?

I personally don't like unfinished stories, so I believe it is necessary - that consciousness is just a side-effect of matter performing some complex computation. It wraps the theory up nicely with a little bow on the top.