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.blp

@@ -0,0 +1,17 @@
+using Gtk 4.0;
+
+Gtk.ListView {
+  factory: Gtk.BuilderListItemFactory list_item_factory {
+    template ListItem {
+      child: Gtk.ListView {
+        factory: Gtk.BuilderListItemFactory list_item_factory {
+          template ListItem {
+            child: Gtk.Label {
+              label: bind template.item as <$MyObject>.name;
+            };
+          }
+        };
+      };
+    }
+  };
+}