Fix unit testing (Airbnb style lint) (vuejs-templates#351)

* Improve unit testing referring the official guide

Unit Testing - vue.js
https://vuejs.org/v2/guide/unit-testing.html

* Allow optionalDependencies

https://google.com/#q=import%2Fno-extraneous-dependencies  'function-bind' should be listed in the project's dependencies, not devDependencies
/project-path/test/unit/index.js
Function.prototype.bind = require('function-bind');
                            ^

Author: inouetakuya

template/.eslintrc.js

@@ -44,6 +44,10 @@ module.exports = {
       'js': 'never',
       'vue': 'never'
     }],
+    // allow optionalDependencies
+    'import/no-extraneous-dependencies': ['error', {
+      'optionalDependencies': ['test/unit/index.js']
+    }],
     {{/if_eq}}
     // allow debugger during development
     'no-debugger': process.env.NODE_ENV === 'production' ? 2 : 0

template/test/unit/specs/Hello.spec.js

@@ -3,10 +3,8 @@ import Hello from 'src/components/Hello'{{#if_eq lintConfig "airbnb"}};{{/if_eq}
 
 describe('Hello.vue', () => {
   it('should render correct contents', () => {
-    const vm = new Vue({
-      el: document.createElement('div'),
-      render: (h) => h(Hello){{#if_eq lintConfig "airbnb"}},{{/if_eq}}
-    }){{#if_eq lintConfig "airbnb"}};{{/if_eq}}
+    const Constructor = Vue.extend(Hello){{#if_eq lintConfig "airbnb"}};{{/if_eq}}
+    const vm = new Constructor().$mount(){{#if_eq lintConfig "airbnb"}};{{/if_eq}}
     expect(vm.$el.querySelector('.hello h1').textContent)
       .to.equal('Welcome to Your Vue.js App'){{#if_eq lintConfig "airbnb"}};{{/if_eq}}
   }){{#if_eq lintConfig "airbnb"}};{{/if_eq}}