test(persist/inventory): add test for adding deferredModule

This feature will allow us to read/write to 'settings' of service
'instances' which are of type deferredModule.
Usually a deferredModule needs to be evaulated, but because we use our
own type, which forces some constraints, we can safely perform read and
write

pkgs/clan-cli/clan_lib/persist/fixtures/deferred.json

@@ -0,0 +1 @@
+{}

pkgs/clan-cli/clan_lib/persist/fixtures/deferred.nix

@@ -0,0 +1,28 @@
+{ clanLib, lib, ... }:
+let
+  eval = lib.evalModules {
+    modules = [
+      {
+        # Trying to write into the default
+        options.foo = lib.mkOption {
+          type = lib.types.attrsOf clanLib.types.uniqueDeferredSerializableModule;
+        };
+      }
+      {
+        foo = {
+          a = { };
+          b = { };
+        };
+      }
+
+      # Merge the "inventory.json"
+      (builtins.fromJSON (builtins.readFile ./deferred.json))
+    ];
+  };
+in
+{
+  clanInternals.inventoryClass.inventory = eval.config;
+  clanInternals.inventoryClass.introspection = clanLib.introspection.getPrios {
+    options = eval.options;
+  };
+}