Draft: Add tests to check if you can swap unique values inside a transaction

The test_swap_unique_together_values_inside_transaction test currently fails.

I've also tried adding the same tests into the 30-integrate-firestore branch (which at the time of this PR is where the implementation of the Firestore connector lives, and which contains a slightly reworked impelementation of the unique constraints), but the same test fails in the same way in that branch too.