Let's see it this way : not mankind is the experiment, but life is.
Mankind is merely an evolutionary curiosity in the experiment of life.
Maybe the God(s) did experiment with life to see where they came from...

"All Knowing" is rather relative : put a genius of our time in the Middle Ages and he'll be seen as a kind of a God. "All Knowing" means : knowing a lot more than we do. Someone who, from our perspective, is all knowing, will still have, in his own perspective, questions on which he wants answers. And maybe "life as we know it" is just one of those experiments to get answers.
Also take into account that we cannot see any further than the boundaries of our own universe. Most common theory is that it originates from the big bang. But what caused the big bang. What was outside of the singularity which exploded? Nobody knows for sure.
In extremis, the singularity which did go big bang migt be a singularity somewhere in a labo of what we call "the Gods"...

You can call me crazy if you want to :-D