feat(Tabbar): add placeholder prop (#5979)

src/nav-bar/test/__snapshots__/index.spec.js.snap

@@ -1,7 +1,7 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`placeholder prop 1`] = `
-<div class="van-nav-bar__placeholder" style="height: 0px;">
+<div class="van-nav-bar__placeholder" style="height: 50px;">
   <div class="van-nav-bar van-nav-bar--fixed van-hairline--bottom">
     <div class="van-nav-bar__left"></div>
     <div class="van-nav-bar__title van-ellipsis"></div>

src/nav-bar/test/index.spec.js

@@ -1,5 +1,5 @@
 import NavBar from '..';
-import { mount } from '../../../test';
+import { mount, mockGetBoundingClientRect } from '../../../test';
 
 test('render left & right slot', () => {
   const wrapper = mount(NavBar, {
@@ -23,6 +23,8 @@ test('render title slot', () => {
 });
 
 test('placeholder prop', () => {
+  const restore = mockGetBoundingClientRect({ height: 50 });
+
   const wrapper = mount(NavBar, {
     propsData: {
       fixed: true,
@@ -31,6 +33,8 @@ test('placeholder prop', () => {
   });
 
   expect(wrapper).toMatchSnapshot();
+
+  restore();
 });
 
 test('click-left event', () => {