[Improvement] Optimize demo test (#1053)

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

@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`renders loading correctly 1`] = `
+exports[`renders demo correctly 1`] = `
 <div>
   <div>
     <div class="van-loading van-loading--circular van-loading--black"><span class="van-loading__spinner van-loading__spinner--circular"> <svg viewBox="25 25 50 50" class="van-loading__circular"><circle cx="50" cy="50" r="20" fill="none"></circle></svg></span></div>

packages/loading/test/demo.spec.js

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