You don't even need to install it. It doesn't take a lot of code to run initdb to create a temporary instance, write a suitable configuration file, launch the postmaster process, and delete the temporary database directory tree after terminating the database after testing is complete. On a not-too-fast Linux system, the time for all that is around half a second. Too much overhead for individual unit tests, but fast enough to run once per test suite run.

Agreed. Running docker-compose and postgres-alpine works just fine.