Duplicate number index tests fix

This will in practice bring us to test parity with 2.0.0 for constraints. Technically, the coverage is still lower than it used to be, pre-2.0.1, but that is because we had some tests that tested for scenarios that I don't think ever actually occur in practice. Specifically, we had tests for populating IndexUpdaters for uniqueness indexes. This is functionality that never gets used because constraint indexes are created synchronously, and are protected by the schema lock, among other things.