systemd: allow custom unit folders to be configured with SYSTEMD_UNIT_PATH

nixos/tests/all-tests.nix

@@ -393,6 +393,7 @@ in
   systemd-networkd-vrf = handleTest ./systemd-networkd-vrf.nix {};
   systemd-nspawn = handleTest ./systemd-nspawn.nix {};
   systemd-timesyncd = handleTest ./systemd-timesyncd.nix {};
+  systemd-unit-path = handleTest ./systemd-unit-path.nix {};
   taskserver = handleTest ./taskserver.nix {};
   telegraf = handleTest ./telegraf.nix {};
   tiddlywiki = handleTest ./tiddlywiki.nix {};

nixos/tests/systemd-unit-path.nix

@@ -0,0 +1,47 @@
+import ./make-test-python.nix ({ pkgs, ... }:
+
+let
+  exampleScript = pkgs.writeTextFile {
+    name = "example.sh";
+    text = ''
+      #! ${pkgs.runtimeShell} -e
+
+      while true; do
+          echo "Example script running" >&2
+          ${pkgs.coreutils}/bin/sleep 1
+      done
+    '';
+    executable = true;
+  };
+
+  unitFile = pkgs.writeTextFile {
+    name = "example.service";
+    text = ''
+      [Unit]
+      Description=Example systemd service unit file
+
+      [Service]
+      ExecStart=${exampleScript}
+
+      [Install]
+      WantedBy=multi-user.target
+    '';
+  };
+in
+{
+  name = "systemd-unit-path";
+
+  machine = { pkgs, lib, ... }: {
+    boot.extraSystemdUnitPaths = [ "/etc/systemd-rw/system" ];
+  };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+    machine.succeed("mkdir -p /etc/systemd-rw/system")
+    machine.succeed(
+        "cp ${unitFile} /etc/systemd-rw/system/example.service"
+    )
+    machine.succeed("systemctl start example.service")
+    machine.succeed("systemctl status example.service | grep 'Active: active'")
+  '';
+})