Add some Fakes for easier interaction snapshot testing

Summary:
`Fake` components are simplified so snapshots are stable and reliable, and references are exported
so that interactions like `onRefresh` and `onScroll` can be called manually. Currently there is just
one global export for each class, but we may change this in the future if we need to manage multiple
`Fake`s of the same type in one render tree.

Right now these must be installed explicitly, but I might move them into `__mocks__` folders if it
seems reasonable to make them defaults.

Reviewed By: cpojer

Differential Revision: D4318207

fbshipit-source-id: 62802353a98b09ca1c80804ef7201ea63091f94a

Libraries/Components/RefreshControl/RefreshControl.js

@@ -152,7 +152,7 @@ const RefreshControl = React.createClass({
     return (
       <NativeRefreshControl
         {...this.props}
-        ref={ref => this._nativeRef = ref}
+        ref={ref => {this._nativeRef = ref;}}
         onRefresh={this._onRefresh}
       />
     );