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 

this._hostContainerInfo = null;
    
    this._updateBatchNumber = null;
    this._pendingElement = null;
    this._pendingStateQueue = null;
    this._pendingReplaceState = false;
    this._pendingForceUpdate = false;
    this._renderedNodeType = null;
    this._renderedComponent = null;
    this._context = null;
    this._mountOrder = 0;
    this._topLevelWrapper = null;
    
    this._pendingCallbacks = null;
    
    this._calledComponentWillUnmount = false;
    if (__DEV__) {
      this._warnedAboutRefsInRender = false;
    }
  },
  /**
   * Initializes the component, renders markup, and registers event listeners.
   *
   * @param {ReactReconcileTransaction|ReactServerRenderingTransaction} transaction
   * @param {?object} hostParent
   * @param {?object} hostContainerInfo
   * @param {?object} context
   * @return {?string} Rendered markup to be inserted into the DOM.
   * @final
   * @internal