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.getName() || 'A component'
      );
      warning(
        typeof inst.componentWillRecieveProps !== 'function',
        '%s has a method called ' +
        'componentWillRecieveProps(). Did you mean componentWillReceiveProps()?',
        (this.getName() || 'A component')
      );
    }
    var initialState = inst.state;
    if (initialState === undefined) {
      inst.state = initialState = null;
    }
    invariant(
      typeof initialState === 'object' && !Array.isArray(initialState),
      '%s.state: must be set to an object or null',
      this.getName() || 'ReactCompositeComponent'
    );
    this._pendingStateQueue = null;
    this._pendingReplaceState = false;
    this._pendingForceUpdate = false;
    var markup;
    if (inst.unstable_handleError) {
      markup = this.performInitialMountWithErrorHandling(
        renderedElement,
        hostParent,
        hostContainerInfo,
        transaction,
        context
      );