Reuse temporary database in local postgres in fake environment (between specific test runs).

[isoos]

• Needed for the SQL migration: a task test checks fallback logic with different runtime version, and it needs to reset all details including cache keys, hence the new scope on each run. This change preserves the backing temp db between such runs.
• The fallback_test.dart change is mostly indent (with the wrapper method).
• Refactored the PrimaryDatabase initialization, I think the new logic has better separation on which cases will trigger a temp db creation and cleanup.