Test which version? Part of the problem is that string theory is a meta theory. There will always be ways to escape any negative experimental result.
String theory isn't a theory it's a family of related theories sharing some common mathematical tools.
People talk about this as though it's an attempt at deception, whereas two people notionally working in string theory could in fact be proposing highly incompatible models which would be conclusively ruled out (and a lot of them have been in so far as that can be done - i.e. experimentation has put tight bounds on their possible parameters).
> There will always be ways to escape any negative experimental result.
Yeah... except that hasn't been proven. That's just your belief, right?
I don't think anyone has proven that string theory can yield no testable predictions. I think if someone had, that'd be a big deal.
And I don't think we should pretend that an open problem is closed just because we don't like it.