rework stirling-pdf to be more generic with test

modules/clan/stirling-pdf/tests/vm/default.nix

@@ -0,0 +1,34 @@
+{
+  ...
+}:
+{
+  name = "service-stirling-pdf";
+
+  clan = {
+    directory = ./.;
+    inventory = {
+      machines.server = { };
+
+      instances = {
+        actual-budget-test = {
+          module.name = "@clan/stirling-pdf";
+          module.input = "self";
+          roles.default.machines."server".settings = { };
+        };
+      };
+    };
+  };
+
+  nodes = {
+    server = { };
+  };
+
+  testScript = ''
+    start_all()
+
+    server.wait_for_unit("stirling-pdf")
+
+    server.succeed("systemctl status stirling-pdf")
+
+  '';
+}