nixos/tests/postgresql-wal-receiver.nix still imported nixos/lib/testing.nix, which is somewhat private API of nixos/tests/make-test.nix, the old perl driver.

Once we remove nixos/tests/make-test.nix, we don't want to keep around nixos/lib/testing.nix.

• This test still needs to be ported to python
• It should probably be updated to just use make-test-python.nix, as most other simple tests
• Does it needs to set a custom postgresqlDataDir? If not, we can remove the tmpfiles hack in the test code, too.