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/ScrollView/__mocks__/ScrollViewMock.js

@@ -0,0 +1,33 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+const React = require('React');
+const View = require('View');
+
+const requireNativeComponent = require('requireNativeComponent');
+
+const RCTScrollView = requireNativeComponent('RCTScrollView');
+
+class ScrollViewMock extends React.Component {
+  render() {
+    return (
+      <RCTScrollView>
+        {this.props.refreshControl}
+        <View>
+          {this.props.children}
+        </View>
+      </RCTScrollView>
+    );
+  }
+}
+
+module.exports = ScrollViewMock;