There is no extra guard or clause in `enqueueForceUpdate` when calling `render` which means the `render` always calls `shouldComponentUpdate` which can block even forced re-render during tests.

This for example [breaks shallow rendering in Enzyme when using MobX and observables](https://github.com/airbnb/enzyme/issues/1267).

Minimal test case:

```javascript
import React from 'react'
import ShallowRenderer from 'react-test-renderer/shallow';
const renderer = new ShallowRenderer();

class TestComponent extends React.Component {
  render() { return <span/> }
}

describe('forceUpdate', () => {
  it('does not call shouldComponentUpdate', () => {
    let called = 0
    TestComponent.prototype.shouldComponentUpdate = () => called += 1
    const component = renderer.render(<TestComponent/>);
    renderer._instance.forceUpdate()
    expect(called).toBe(0) // <- this throws because called === 1
  })
})
```
state = {update: false};
      shouldComponentUpdate(nextProps, nextState) {
        return this.state.update !== nextState.update;
      }
      render() {
        renderCounter++;
        return <div>{`${renderCounter}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({update: false});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    instance.setState({update: true});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>2</div>);
  });
  it('should not run shouldComponentUpdate during forced update', () => {
    let scuCounter = 0;
    class SimpleComponent extends React.Component {