[Improvement] Optimize demo test (#1053)

packages/datetime-picker/test/__snapshots__/demo.spec.js.snap

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

packages/datetime-picker/test/demo.spec.js

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

packages/datetime-picker/test/index.spec.js

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