Remove use of the deprecated string type

tests/minimal.nix

@@ -1,31 +0,0 @@
-#  nixos-mailserver: a simple mail server
-#  Copyright (C) 2016-2018  Robin Raymond
-#
-#  This program is free software: you can redistribute it and/or modify
-#  it under the terms of the GNU General Public License as published by
-#  the Free Software Foundation, either version 3 of the License, or
-#  (at your option) any later version.
-#
-#  This program is distributed in the hope that it will be useful,
-#  but WITHOUT ANY WARRANTY; without even the implied warranty of
-#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-#  GNU General Public License for more details.
-#
-#  You should have received a copy of the GNU General Public License
-#  along with this program. If not, see <http://www.gnu.org/licenses/>
-
-import <nixpkgs/nixos/tests/make-test.nix> {
-
-  machine =
-    { config, pkgs, ... }:
-    {
-        imports = [
-            ./../default.nix
-        ];
-    };
-
-  testScript =
-    ''
-      $machine->waitForUnit("multi-user.target");
-    '';
-}