fix UT; review by qow

Author: hechangqun

test/maplibregl/overlay/ThreeLayerSpec.js

@@ -177,6 +177,7 @@ describe('maplibregl_ThreeLayer', () => {
 
  it('render _update draw', () => {
  expect(threeLayer.render()).not.toBeNull();
- expect(threeLayer.draw()).not.toBeNull();
+ const _this = threeLayer._update();
+ expect(_this.draw()).not.toBeNull();
  });
 });