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
  })
})
```
<span className="child2" />
        </div>
      );
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent foo={'FOO'} />, {
      bar: 'BAR',
    });
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <div>FOO</div>,
      <div>BAR</div>,
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should shallow render a component returning strings directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value="foo" />);
    expect(result).toEqual('foo');
  });
  it('should shallow render a component returning numbers directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value={10} />);
    expect(result).toEqual(10);
  });
  it('should shallow render a fragment', () => {
    class SomeComponent extends React.Component {