Function update

packages/react-test-renderer/src/ReactTestRenderer.js:579–584

(newElement: React$Element<any>)

      return toTree(root.current);
    },
    update(newElement: React$Element<any>): number | void {
      if (root == null || root.current == null) {
        return;
      }
      updateContainer(newElement, root, null, null);
    },
    unmount() {
      if (root == null || root.current == null) {
        return;