Update template tests to follow all of @gaearon's excellent advice.

Author: anp

react-native-scripts/src/scripts/init.js

@@ -38,7 +38,7 @@ module.exports = async (appPath: string, appName: string, verbose: boolean) => {
     eject: 'react-native-scripts eject',
     android: 'react-native-scripts android',
     ios: 'react-native-scripts ios',
-    test: 'node node_modules/jest/bin/jest.js',
+    test: 'node node_modules/jest/bin/jest.js --watch',
   };
 
   appPackage.jest = {

react-native-scripts/template/App.test.js

@@ -0,0 +1,11 @@
+import React from "react";
+import ReactNative from "react-native";
+import App from "./App";
+
+// this is included from the jest-expo preset
+// make sure to include it in package.json yourself if you change the jest preset
+import renderer from "react-test-renderer";
+
+it("renders without crashing", () => {
+  const tree = renderer.create(<App />);
+});