Ok so firstly, no theory can be proven. You're thinking of theorems. One of the tests of a scientific theory is its falsifiability. A simple example would be that if a single apple floats upwards from the tree instead of falling to earth, that would falsify the theory of gravity. In string theory, the falsifiability lies in the predictions of quantum mechanics. A falsification of QM would collapse string theory immediately. Of course, you've chosen this particular theory because it is at the fringes of current understanding and there is debate over whether it's a legitimate theory. However, it is actually founded on rigorous study, and its predictions are exactly as consequential as its falsifiability is agreed upon.
While it cannot be proven using current methodologies, that problem puts it in such distinguished company as general relativity and even Galilean relativity were in terms of the experimental technology available to natural philosophers at the times of conception. String theory can be proved, or disproved, just not yet. So we don't write it off as academic, but we file it under "pending", until such a time as we are able to test it. This is absolutely the astute thing to do.
If you have a test for God, please propose it. It seems that this particular question is beyond both practical and philosophical technology at this point in human history. There is no theory about God that can be tested, falsified, repeated and scrutinised as far as I know, so why would science waste time on this question? Maybe in the future we will have a knife of some kind that can carve meaning into this question, but we don't at the moment. There's a separate discipline for pondering abstract questions which we can't test, it's called philosophy. And it pushes science, when the time is right, to find evidence. But until philosophers find a way to test their suppositions, their suppositions are not worth thinking about,for a scientist.