gsasl: export LOCALDOMAIN before running tests

Recently kerberos added a code path that relies ont he local domain part
to be available. See [hydra] for the recent build failure and [github]
for a brief analysis of the error.

[hydra] https://hydra.nixos.org/build/113717403
[github]:
  - https://github.com/NixOS/nixpkgs/pull/81531#issuecomment-593840860
  - https://github.com/NixOS/nixpkgs/pull/81531#issuecomment-593853565