postgresdsn: add test for envvars we generate on NixOS

Created by: keegancsmith

This is a test case to ensure we don't break how we connect to databases for people using our nix hooks. On nix we setup a postgres which is exposed via a unix socket. The test case we added includes an example set of envvars we create and ensures we get out the correct DSN.

This recently broke for our test environments but not in this code. This is just making sure we don't accidently break it as we confidently refactor :)