Django 5.1 removes support for Meta.index_together in favor of just

including these in Meta.indexes. A previous change updated signature

calculation and existing unit tests for this, but one thing I wanted to

make sure continued to work was an evolution which removes

index_together and adds indexes, even when running on Django>=5.1.
This change adds a test which will manually modify the signature (on

affected versions) to simulate this case, and verify that we end up

correctly modifying the signature while generating empty SQL (since

there's no change in the actual indexes).

Ran new test on all versions and database types.