feat(NavBar): add placeholder prop (#5938)

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

@@ -15,7 +15,7 @@ exports[`renders demo correctly 1`] = `
       <div class="van-nav-bar__left"><i class="van-icon van-icon-arrow-left van-nav-bar__arrow">
           <!----></i><span class="van-nav-bar__text">返回</span></div>
       <div class="van-nav-bar__title van-ellipsis">标题</div>
-      <div class="van-nav-bar__right"><i class="van-icon van-icon-search">
+      <div class="van-nav-bar__right"><i class="van-icon van-icon-search" style="font-size: 18px;">
           <!----></i></div>
     </div>
   </div>

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

@@ -1,5 +1,15 @@
 // 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 van-nav-bar--fixed van-hairline--bottom">
+    <div class="van-nav-bar__left"></div>
+    <div class="van-nav-bar__title van-ellipsis"></div>
+    <div class="van-nav-bar__right"></div>
+  </div>
+</div>
+`;
+
 exports[`render left & right slot 1`] = `
 <div class="van-nav-bar van-hairline--bottom">
   <div class="van-nav-bar__left">Custom Left</div>

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

@@ -21,3 +21,36 @@ test('render title slot', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('placeholder prop', () => {
+  const wrapper = mount(NavBar, {
+    propsData: {
+      fixed: true,
+      placeholder: true,
+    },
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
+test('click-left event', () => {
+  const wrapper = mount(NavBar, {
+    propsData: {
+      leftText: 'left',
+    },
+  });
+
+  wrapper.find('.van-nav-bar__left').trigger('click');
+  expect(wrapper.emitted('click-left')).toBeTruthy();
+});
+
+test('click-right event', () => {
+  const wrapper = mount(NavBar, {
+    propsData: {
+      rightText: 'right',
+    },
+  });
+
+  wrapper.find('.van-nav-bar__right').trigger('click');
+  expect(wrapper.emitted('click-right')).toBeTruthy();
+});