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

Created by  danbst  on March 29, 2019
3HRBFWGK2JHMIV76HEF52DEJPMXFW6W6LL3ISFVPREPOW7AYHX3QC
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