fix(Button): icon-prefix prop not work (#5947)

src/button/test/__snapshots__/index.spec.js.snap

@@ -1,6 +1,11 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`loading size 1`] = `
+exports[`icon-prefix prop 1`] = `
+<button class="van-button van-button--default van-button--normal"><i class="my-icon my-icon-success van-button__icon">
+    <!----></i></button>
+`;
+
+exports[`loading-size prop 1`] = `
 <button class="van-button van-button--default van-button--normal van-button--loading">
   <div class="van-loading van-loading--circular van-button__loading"><span class="van-loading__spinner van-loading__spinner--circular" style="color: currentColor; width: 10px; height: 10px;"><svg viewBox="25 25 50 50" class="van-loading__circular"><circle cx="50" cy="50" r="20" fill="none"></circle></svg></span></div>
 </button>

src/button/test/index.spec.js

@@ -1,7 +1,7 @@
 import { mount } from '../../../test';
 import Button from '..';
 
-test('loading size', () => {
+test('loading-size prop', () => {
   const wrapper = mount(Button, {
     propsData: {
       loading: true,
@@ -82,3 +82,14 @@ test('hide border when color is gradient', () => {
 
   expect(wrapper.element.style.border).toEqual('0px');
 });
+
+test('icon-prefix prop', () => {
+  const wrapper = mount(Button, {
+    propsData: {
+      icon: 'success',
+      iconPrefix: 'my-icon',
+    },
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});