**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.
render() {
        return React.createElement('div', null, this.props.name);
      }
    }
    SimpleComponent.propTypes = {
      name: PropTypes.string.isRequired,
    };
    const shallowRenderer = createRenderer();
    shallowRenderer.render(React.createElement(SimpleComponent, {name: 123}));
    if (__DEV__) {
      expect(console.error.calls.count()).toBe(1);
      expect(
        console.error.calls.argsFor(0)[0].replace(/\(at .+?:\d+\)/g, '(at **)'),
      ).toBe(
        'Warning: Failed prop type: Invalid prop `name` of type `number` ' +
          'supplied to `SimpleComponent`, expected `string`.\n' +
          '    in SimpleComponent',
      );
    }
  });
  it('should enable rendering of cloned element', () => {
    class SimpleComponent extends React.Component {