Fix gltf serializer tests

serializers/src/glTF/2.0/index.ts

@@ -4,4 +4,5 @@ export * from "./glTFExporter";
 export * from "./glTFExporterExtension";
 export * from "./glTFMaterialExporter";
 export * from "./glTFSerializer";
-export * from "./glTFUtilities";
+export * from "./glTFUtilities";
+export * from "./Extensions";