> But then how do you deal with unique constraints or things like upserts? If I want to add 10 entries, then I need to know that all have been added and not pre-existing, or else the code for adding new entries doesn't actually get tested.

Then your test code needs to take into account the current state of the database when creating input data for the code under test. It is more complicated than with a know start state where you can pick fixed data, sure, but it's doable.