Fix tests

Libraries/Utilities/__mocks__/PixelRatio.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright 2004-present Facebook. All Rights Reserved.
+ */
+'use strict';
+
+var PixelRatio = {
+  startDetecting: function () {
+    // noop for our implementation
+  },
+
+  get: function() {
+    return 2;
+  },
+
+  getPixelSizeForLayoutSize: function (layoutSize) {
+    return Math.round(layoutSize * PixelRatio.get());
+  }
+};
+
+module.exports = PixelRatio;