Redid some of the ignored UniqueIndexAccessorCompatibility tests in term...

...s of UniqueConstraintCompatibility.

The populating IndexUpdater test, however, is not equivalent to the test it replaces, because our
pessimistic concurrency control seems to make some of those particular scenarios in the old tests,
impossible to hit through the public API.

The work of rewriting the tests is not yet done. This is just a step of the way.

---

Rebased and retargeted version of #1896