USD: Add additional test for scene instancing Import

This pairs with the recently added Export test and reuses the same
.blend file. While there happens to be an instancing related rendering
test, that happens to cover Import, it's better to have a direct test
for it.

Future improvements to both Export and Import to account for the recent
"hidden layer" visibility bug would be good to do as a follow-up.

Pull Request: https://projects.blender.org/blender/blender/pulls/133782

tests/python/bl_usd_import_test.py

@@ -1749,6 +1749,36 @@ class USDImportTest(AbstractUSDTest):
         root = bpy.data.objects["root"]
         self.assertEqual(self.round_vector(root.scale), [1.0, 1.0, 1.0])
 
+    def test_import_native_instancing(self):
+        """Test importing USD files with scene instancing."""
+
+        # Use the existing instancing test file to create the USD file
+        # for import. It is validated as part of the bl_usd_export test.
+        bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "nested_instancing_test.blend"))
+
+        testfile = str(self.tempdir / "usd_export_nested_instancing_true.usda")
+        res = bpy.ops.wm.usd_export(filepath=testfile, use_instancing=True)
+        self.assertEqual({'FINISHED'}, res, f"Unable to export to {testfile}")
+
+        # Reload the empty file and import back in
+        bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "empty.blend"))
+        res = bpy.ops.wm.usd_import(filepath=testfile)
+        self.assertEqual({'FINISHED'}, res, f"Unable to import USD file {testfile}")
+
+        # If instancing is working there should only be 1 real mesh and 1 real point cloud
+        self.assertEqual(len(bpy.data.meshes), 1)
+        self.assertEqual(len(bpy.data.pointclouds), 1)
+        real_names = [bpy.data.meshes[0].name, bpy.data.pointclouds[0].name]
+
+        # There should be 6 instances found for the above real objects (not counting empties etc.)
+        instance_count = 0
+        depsgraph = bpy.context.evaluated_depsgraph_get()
+        for object_instance in depsgraph.object_instances:
+            if object_instance.is_instance and object_instance.object.name in real_names:
+                instance_count += 1
+
+        self.assertEqual(instance_count, 6, "Unexpected number of instances found")
+
     def test_material_import_usd_hook(self):
         """Test importing color from an mtlx shader."""