[Improvement] Optimize demo test (#1053)

packages/pull-refresh/test/__snapshots__/demo.spec.js.snap

@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`renders pull-refresh correctly 1`] = `
+exports[`renders demo correctly 1`] = `
 <div>
   <div class="van-pull-refresh">
     <div class="van-pull-refresh__track" style="transition:0ms;transform:translate3d(0,0px, 0);">

packages/pull-refresh/test/demo.spec.js

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

packages/pull-refresh/test/index.spec.js

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