feat(Image): add loading-icon prop (#5469)

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

@@ -25,6 +25,13 @@ exports[`load event 2`] = `
 </div>
 `;
 
+exports[`loading-icon prop 1`] = `
+<div class="van-image"><img class="van-image__img">
+  <div class="van-image__loading"><i class="van-icon van-icon-success van-image__loading-icon">
+      <!----></i></div>
+</div>
+`;
+
 exports[`radius prop 1`] = `
 <div class="van-image" style="overflow: hidden; border-radius: 3px;"><img src="https://img.yzcdn.cn/vant/cat.jpeg" class="van-image__img">
   <div class="van-image__loading"><i class="van-icon van-icon-photo-o van-image__loading-icon">

src/image/test/index.spec.js

@@ -125,6 +125,16 @@ test('show-error prop', () => {
   expect(wrapper).toMatchSnapshot();
 });
 
+test('loading-icon prop', () => {
+  const wrapper = mount(Image, {
+    propsData: {
+      loadingIcon: 'success'
+    }
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
 test('radius prop', () => {
   const wrapper = mount(Image, {
     propsData: {