Change test scene, update reference picture

tests/validation/config.json

@@ -821,7 +821,7 @@
         },
         {
             "title": "GLTF Extension EXT_mesh_gpu_instancing",
-            "playgroundId": "#QFIGLW#4",
+            "playgroundId": "#QFIGLW#5",
             "referenceImage": "gltfExtensionExtMeshGpuInstancingTest.png"
         }
     ]