[backport 22.11] tests: fix gnupg stub (#3702)

systemd now depends on `gnupg.override`, so we need a stub for systemd too.

(cherry picked from commit 7ff468105d)

tests/modules/programs/gpg/mutable-keyfiles.nix

@@ -14,6 +14,7 @@
   };
 
   test.stubs.gnupg = { };
+  test.stubs.systemd = { }; # depends on gnupg.override
 
   nmt.script = ''
     assertFileContains activate "export GNUPGHOME='/home/hm-user/.gnupg'"

tests/modules/services/gpg-agent/default-homedir.nix

@@ -9,6 +9,7 @@ with lib;
     programs.gpg.enable = true;
 
     test.stubs.gnupg = { };
+    test.stubs.systemd = { }; # depends on gnupg.override
 
     nmt.script = ''
       in="${config.systemd.user.sockets.gpg-agent.Socket.ListenStream}"

tests/modules/services/gpg-agent/override-homedir.nix

@@ -12,6 +12,7 @@ with lib;
     };
 
     test.stubs.gnupg = { };
+    test.stubs.systemd = { }; # depends on gnupg.override
 
     nmt.script = ''
       in="${config.systemd.user.sockets.gpg-agent.Socket.ListenStream}"