[Improvement] Optimize demo test (#1053)

packages/badge/test/__snapshots__/demo.spec.js.snap

@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`renders badge correctly 1`] = `
+exports[`renders demo correctly 1`] = `
 <div>
   <div>
     <div class="van-hairline--top-bottom van-badge-group">

packages/badge/test/demo.spec.js

@@ -0,0 +1,4 @@
+import Demo from '../demo';
+import demoTest from '../../../test/demo-test';
+
+demoTest(Demo);

packages/badge/test/index.spec.js

@@ -1,9 +1,6 @@
 import { mount } from '@vue/test-utils';
 import Badge from '../';
 import BadgeGroup from '../../badge-group';
-import demoTest from '../../../test/demo-test';
-
-demoTest(Badge);
 
 test('click', () => {
   const onClick = jest.fn();