tests: change postgresql socket dir to /tmp

In https://github.com/NixOS/nixpkgs/pull/57677 default postgresql socket directory was changed
to `/run/postgresql`, which doesn't exist (and can't be created) in
Nix build environment. We'll use /tmp as socket dir explicitly then.

Fixes build failure https://hydra.nixos.org/build/91221682

Cc @aszlig @edolstra

.gitignore

@@ -32,3 +32,4 @@ Makefile.in
 /inst
 hydra-config.h
 hydra-config.h.in
+result