[Improvement] Optimize demo test (#1053)

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

@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`renders circle correctly 1`] = `
+exports[`renders demo correctly 1`] = `
 <div>
   <div>
     <div class="van-circle" style="width:120px;height:120px;">

packages/circle/test/demo.spec.js

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