[improvement] ImagePreview: add fade transition (#2196)

packages/image-preview/test/__snapshots__/index.spec.js.snap

@@ -1,7 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`render image 1`] = `
-<div class="van-image-preview">
+<div class="van-image-preview" name="van-fade">
   <div class="van-image-preview__index">
     1/3
   </div>
@@ -23,7 +23,7 @@ exports[`render image 1`] = `
 `;
 
 exports[`zoom 1`] = `
-<div class="van-image-preview">
+<div class="van-image-preview" name="van-fade">
   <div class="van-image-preview__index">
     1/3
   </div>

packages/image-preview/test/index.spec.js

@@ -1,7 +1,9 @@
 import Vue from 'vue';
 import ImagePreview from '..';
 import ImagePreviewVue from '../ImagePreview';
-import { mount, trigger, triggerDrag } from '../../../test/utils';
+import { mount, trigger, triggerDrag, transitionStub } from '../../../test/utils';
+
+transitionStub();
 
 function triggerZoom(el, x, y) {
   trigger(el, 'touchstart', 0, 0, { x, y });