Add static index based backcompat tests

I'm looking for initial feedback on this approach to solve #8065. I refactored in such a way that the upgrade tests can use the same framework.  I am currently still trying to get 0.90 indexes to build correctly (explicit flush doesn't seem to work?), but I think the basics are all here.

closes #8065
I was in the process of changing to use this method which was added to ElasticsearchIntegrationTest and forgot to remove.  Fixed.