pg_mockable is one of the spin-off PostgreSQL extensions that I’ve created for the backend of our high-performance MQTT hosting platform: FlashMQ. It’s basic premise is that you (through one of the helper function that the extension provides) create a mockable wrapper function around a function that you want to mock. That wrapper function will be a one-statement SQL function, so that in many cases it will be inlineable.
Then, when during testing you need to mock (for instance now() and its entire family), you can call the mockable.mock() function, which will replace that wrapper function to return the second argument fed to mockable.mock(), until mock.unmock() is called to restore the original wrapper function.
pg_mockable is now also available via the PostgreSQL Extension Network (PGXN): https://pgxn.org/dist/pg_mockable/