nixos.tests.systemd-journal: add basic systemd-journal-gatewayd test

This test allows to ensure the systemd-journal-gatewayd service is
responding correcly when the NixOS option `enableHttpGateway` is set.

The test has not been added into the main systemd test because a
graphical stack is not required (and rebuilding the graphical stack on
systemd change is huge).

nixos/tests/all-tests.nix

@@ -340,6 +340,7 @@ in
   systemd-binfmt = handleTestOn ["x86_64-linux"] ./systemd-binfmt.nix {};
   systemd-boot = handleTest ./systemd-boot.nix {};
   systemd-confinement = handleTest ./systemd-confinement.nix {};
+  systemd-journal = handleTest ./systemd-journal.nix {};
   systemd-timesyncd = handleTest ./systemd-timesyncd.nix {};
   systemd-networkd-vrf = handleTest ./systemd-networkd-vrf.nix {};
   systemd-networkd = handleTest ./systemd-networkd.nix {};

nixos/tests/systemd-journal.nix

@@ -0,0 +1,20 @@
+import ./make-test-python.nix ({ pkgs, ... }:
+
+{
+  name = "systemd-journal";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ lewo ];
+  };
+
+  machine = { pkgs, lib, ... }: {
+    services.journald.enableHttpGateway = true;
+  };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+
+    machine.succeed(
+        "${pkgs.curl}/bin/curl -s localhost:19531/machine | ${pkgs.jq}/bin/jq -e '.hostname == \"machine\"'"
+    )
+  '';
+})