Heh... as someone on the outside, I feel the need to ask:
Has it been rigorously shown that it can never be tested? Or is that your prediction?
They are getting close to making a testable prediction, any day now. Have been for the last 30 years. History is not always an indication of the future, but it is often a good sign.
But yes, not rigorous.
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.