[build] optimize demo test

test/demo-test.ts

@@ -1,10 +1,12 @@
 import Vue from 'vue';
 import '../docs/src/demo-common';
 import Locale from '../packages/locale';
-import { renderToString } from '@vue/server-test-utils';
+import { mount, later } from './utils';
 
 const empty = {
-  template: '<div><slot></slot></div>',
+  render(h) {
+    return h('div', [this.$slots.default]);
+  },
   inheritAttrs: false
 };
 
@@ -12,11 +14,15 @@ Vue.component('demo-block', empty);
 Vue.component('demo-section', empty);
 
 export default function (Demo: any) {
-  test('renders demo correctly', () => {
+  test('renders demo correctly', async () => {
     if (Demo.i18n) {
       Locale.add(Demo.i18n);
     }
 
-    expect(renderToString(Demo)).toMatchSnapshot();
+    const wrapper = mount(Demo);
+
+    await later();
+
+    expect(wrapper).toMatchSnapshot();
   });
 }