add some api

test/unit/specs/HelloWorld.spec.js

@@ -0,0 +1,11 @@
+import Vue from 'vue'
+import HelloWorld from '@/components/HelloWorld'
+
+describe('HelloWorld.vue', () => {
+  it('should render correct contents', () => {
+    const Constructor = Vue.extend(HelloWorld)
+    const vm = new Constructor().$mount()
+    expect(vm.$el.querySelector('.hello h1').textContent)
+      .toEqual('Welcome to Your Vue.js App')
+  })
+})