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
  })
})
```
static contextTypes = {
        foo: PropTypes.string,
      };
      state = {
        bar: 'bar',
      };
      render() {
        return <div>{`${this.context.foo}:${this.state.bar}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent />, {
      foo: 'foo',
    });
    expect(result).toEqual(<div>foo:bar</div>);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({bar: 'baz'});
    result = shallowRenderer.getRenderOutput();
    expect(result).toEqual(<div>foo:baz</div>);
  });
  it('can fail context when shallowly rendering', () => {
    spyOn(console, 'error');
    class SimpleComponent extends React.Component {