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 

} else {
      markup = this.performInitialMount(renderedElement, hostParent, hostContainerInfo, transaction, context);
    }
    if (inst.componentDidMount) {
      if (__DEV__) {
        transaction.getReactMountReady().enqueue(() => {
          measureLifeCyclePerf(
            () => inst.componentDidMount(),
            this._debugID,
            'componentDidMount'
          );
        });
      } else {
        transaction.getReactMountReady().enqueue(inst.componentDidMount, inst);
      }
    }
    return markup;
  },
  _constructComponent: function(
    doConstruct,
    publicProps,
    publicContext,
    updateQueue
  ) {
    if (__DEV__) {
      ReactCurrentOwner.current = this;
      try {
        return this._constructComponentWithoutOwner(
          doConstruct,
          publicProps,