**Do you want to request a *feature* or report a *bug*?**
report a bug

**What is the current behavior?**
When I call setState with a callback in a test using react shallow renderer (via enzyme), the callback gets called and `this.state` is still the old state.

EDIT: This seems limited to componentWillMount

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).**

EDIT: Reproduce with this https://github.com/bdwain/setstate-callback-bug

**What is the expected behavior?**
When the setState callback gets called, it should have access to the new state.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Version 16.
It worked in v15 with enzyme 2.

EDIT: I think this is because in componentWillMount, [this line](https://github.com/facebook/react/blob/master/packages/react-test-renderer/src/ReactShallowRenderer.js#L67) causes the render method in the shallow wrapper to return early, before it updates the state.
componentDidMount() {
        this.setState({}, (...args) => (mockArgs = args));
      }
      render() {
        return false;
      }
    }
    ReactTestUtils.renderIntoDocument(<Component />);
    expect(mockArgs.length).toEqual(0);
  });
  it('should return a meaningful warning when constructor is returned', () => {
    spyOnDev(console, 'error');
    class RenderTextInvalidConstructor extends React.Component {
      constructor(props) {
        super(props);
        return {something: false};
      }
      render() {