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() || 'ReactCompositeComponent'
    );
    var willReceive = false;
    var nextContext;
    
    if (this._context === nextUnmaskedContext) {
      nextContext = inst.context;
    } else {
      nextContext = this._processContext(nextUnmaskedContext);
      willReceive = true;
    }
    var prevProps = prevParentElement.props;
    var nextProps = nextParentElement.props;
    
    if (prevParentElement !== nextParentElement) {
      willReceive = true;
    }
    
    
    
    if (willReceive && inst.componentWillReceiveProps) {
      if (__DEV__) {
        measureLifeCyclePerf(
          () => inst.componentWillReceiveProps(nextProps, nextContext),
          this._debugID,
          'componentWillReceiveProps',
        );
      } else {
        inst.componentWillReceiveProps(nextProps, nextContext);
      }