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 

publicContext,
          updateQueue
        );
      } finally {
        ReactCurrentOwner.current = null;
      }
    } else {
      return this._constructComponentWithoutOwner(
        doConstruct,
        publicProps,
        publicContext,
        updateQueue
      );
    }
  },
  _constructComponentWithoutOwner: function(
    doConstruct,
    publicProps,
    publicContext,
    updateQueue
  ) {
    var Component = this._currentElement.type;
    if (doConstruct) {
      if (__DEV__) {
        return measureLifeCyclePerf(
          () => new Component(publicProps, publicContext, updateQueue),
          this._debugID,
          'ctor'
        );
      } else {