Enable jest testing

__tests__/index.android.js

+import 'react-native';
+import React from 'react';
+// Note: test renderer must be required after react-native.
+import renderer from 'react-test-renderer';
+
+import Index from '../app/containers/app';
+
+it('renders correctly', () => {
+  const tree = renderer.create(
+    <Index />,
+  );
+});

__tests__/index.ios.js

+import 'react-native';
+import React from 'react';
+// Note: test renderer must be required after react-native.
+import renderer from 'react-test-renderer';
+
+import Index from '../app/containers/app';
+
+it('renders correctly', () => {
+  const tree = renderer.create(
+    <Index />,
+  );
+});