rework stirling-pdf to be more generic with test

modules/clan/stirling-pdf/flake-module.nix

@@ -0,0 +1,19 @@
+{ lib, ... }:
+let
+  module = lib.modules.importApply ./default.nix { };
+in
+{
+  clan.modules = {
+    stirling-pdf = module;
+  };
+
+  perSystem =
+    { ... }:
+    {
+      clan.nixosTests.stirling-pdf = {
+        imports = [ ./tests/vm/default.nix ];
+
+        clan.modules."@clan/stirling-pdf" = module;
+      };
+    };
+}