nixos/tests/hostname: check system's host name

NixOS currently has issues with setting the FQDN of a system in a way
where standard tools work. In order to help with experimentation and
avoid regressions, add a test that checks that the hostname is
reported as the user wanted it to be.

nixos/tests/all-tests.nix

@@ -131,6 +131,7 @@ in
   hitch = handleTest ./hitch {};
   hocker-fetchdocker = handleTest ./hocker-fetchdocker {};
   home-assistant = handleTest ./home-assistant.nix {};
+  hostname = handleTest ./hostname.nix {};
   hound = handleTest ./hound.nix {};
   hydra = handleTest ./hydra {};
   hydra-db-migration = handleTest ./hydra/db-migration.nix {};

nixos/tests/hostname.nix

@@ -0,0 +1,50 @@
+{ system ? builtins.currentSystem,
+  config ? {},
+  pkgs ? import ../.. { inherit system config; }
+}:
+
+with import ../lib/testing-python.nix { inherit system pkgs; };
+with pkgs.lib;
+
+let
+  makeHostNameTest = hostName: domain:
+    let
+      fqdn = hostName + (optionalString (domain != null) ".${domain}");
+    in
+      makeTest {
+        name = "hostname-${fqdn}";
+
+        machine = { lib, ... }: {
+          networking.hostName = hostName;
+          networking.domain = domain;
+
+          environment.systemPackages = with pkgs; [
+            inetutils
+          ];
+        };
+
+        testScript = ''
+          start_all()
+
+          machine = ${hostName}
+
+          machine.wait_for_unit("network.target")
+          assert "${fqdn}" == machine.succeed("hostname --fqdn").strip()
+          assert "${optionalString (domain != null) domain}" == machine.succeed("dnsdomainname").strip()
+          assert (
+              "${fqdn}"
+              == machine.succeed(
+                  'hostnamectl status | grep "Static hostname" | cut -d: -f2'
+              ).strip()
+          )
+        '';
+      };
+
+in
+{
+  noExplicitDomain = makeHostNameTest "ahost" null;
+
+  # Fails due to #10183
+  # explicitDomain = makeHostNameTest "ahost" "adomain";
+}
+