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 (this._pendingStateQueue) {
        inst.state = this._processPendingState(inst.props, inst.context);
      }
    }
    
    if (renderedElement === undefined) {
      renderedElement = this._renderValidatedComponent();
    }
    var nodeType = ReactNodeTypes.getType(renderedElement);
    this._renderedNodeType = nodeType;
    var child = this._instantiateReactComponent(
      renderedElement,
      nodeType !== ReactNodeTypes.EMPTY 
    );
    this._renderedComponent = child;
    var markup = ReactReconciler.mountComponent(
      child,
      transaction,
      hostParent,
      hostContainerInfo,
      this._processChildContext(context),
      debugID
    );
    if (__DEV__) {
      if (debugID !== 0) {
        var childDebugIDs = child._debugID !== 0 ? [child._debugID] : [];
        ReactInstrumentation.debugTool.onSetChildren(debugID, childDebugIDs);
      }
    }
    return markup;