Add tests for nested templates

Basically just a copy of the list_factory test, but with an extra copy
of the list factory inside it.

tests/samples/list_factory_nested_dec.blp

@@ -0,0 +1,18 @@
+using Gtk 4.0;
+
+ListView {
+  factory: BuilderListItemFactory list_item_factory {
+    template ListItem {
+      child: ListView {
+        factory: BuilderListItemFactory list_item_factory {
+          template ListItem {
+            child: Label {
+              label: bind template.item as <$MyObject>.name;
+            };
+          }
+        };
+      };
+    }
+  };
+}
+