Fix upgrade logic to check for major version bump.

I had not noticed this since I was only testing using bwc tests, which only go back to ES 1.1 (which is still lucene 4.x).
The scope is TEST, so that more tests can be added for "old" indexes.  The name is purposefully general, so that any tests for indexes older than can be tested in normal backcompat tests can be tested (assuming this eventually is worked out to a general test class or something).