I would just create a per-test-file database (and delete it the next time the test runs). The overhead is very small (compared to booting Postgres) and it works exactly like production because it is exactly production.

In general, I am not comfortable using an "almost equivalent" database for tests. Every database engine has its quirks, and if you don't know what they are, your program that passes its tests only "almost" works in production, which is annoying.