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
  })
})
```
render() {
        return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    expect(() => shallowRenderer.render(SomeComponent)).toThrowError(
      'ReactShallowRenderer render(): Invalid component element. Instead of ' +
        'passing a component class, make sure to instantiate it by passing it ' +
        'to React.createElement.',
    );
    expect(() => shallowRenderer.render(<div />)).toThrowError(
      'ReactShallowRenderer render(): Shallow rendering works only with ' +
        'custom components, not primitives (div). Instead of calling ' +
        '`.render(el)` and inspecting the rendered output, look at `el.props` ' +
        'directly instead.',
    );
  });
  it('should have shallow unmounting', () => {
    const componentWillUnmount = jest.fn();
    class SomeComponent extends React.Component {
      componentWillUnmount = componentWillUnmount;
      render() {