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 

return new Component(publicProps, publicContext, updateQueue);
      }
    }
    
    
    if (__DEV__) {
      return measureLifeCyclePerf(
        () => Component(publicProps, publicContext, updateQueue),
        this._debugID,
        'render'
      );
    } else {
      return Component(publicProps, publicContext, updateQueue);
    }
  },
  performInitialMountWithErrorHandling: function(
    renderedElement,
    hostParent,
    hostContainerInfo,
    transaction,
    context
  ) {
    var markup;
    var checkpoint = transaction.checkpoint();
    try {
      markup = this.performInitialMount(renderedElement, hostParent, hostContainerInfo, transaction, context);
    } catch (e) {
      
      transaction.rollback(checkpoint);
      this._instance.unstable_handleError(e);