If you attach a `ref` to an element using a string (`ref='foo'`) and use `ReactTestRenderer` you get the `"Stateless function components cannot be given refs"` warning

See https://github.com/facebook/react/issues/7371#issuecomment-237816352

This is because `getPublicInstance` returns `null` and `attachRef` requires that the instance be non-null in the warning invariant.

I think we can either return a simple instance instead of `null` or use some additional check in `attachRef` to see if we're dealing with `ReactTestRenderer`. The first option seems reasonable, given the comment in the current code: 

``` js
ReactTestComponent.prototype.getPublicInstance = function() {
  // I can't say this makes a ton of sense but it seems better than throwing.
  // Maybe we'll revise later if someone has a good use case.
  return null;
};
```

cc @spicyj 

if (__DEV__) {
      renderedComponent = measureLifeCyclePerf(
        () => inst.render(),
        this._debugID,
        'render'
      );
    } else {
      renderedComponent = inst.render();
    }
    if (__DEV__) {
      
      if (renderedComponent === undefined &&
          inst.render._isMockFunction) {
        
        
        renderedComponent = null;
      }
    }
    return renderedComponent;
  },
  /**
   * @private
   */
  _renderValidatedComponent: function() {
    var renderedComponent;
    if (__DEV__ || this._compositeType !== CompositeTypes.StatelessFunctional) {
      ReactCurrentOwner.current = this;
      try {
        renderedComponent =
          this._renderValidatedComponentWithoutOwnerOrContext();