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
  })
})
```
return <div><Child /></div>;
    }
    function Child() {
      throw Error('This component should not render');
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(React.createElement(Parent));
  });
  it('should have shallow rendering', () => {
    class SomeComponent extends React.Component {
      render() {
        return (
          <div>
            <span className="child1" />
            <span className="child2" />
          </div>
        );
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent />);
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should enable shouldComponentUpdate to prevent a re-render', () => {
    let renderCounter = 0;
    class SimpleComponent extends React.Component {