test(Button): add basic test case

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

@@ -0,0 +1,13 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`loading-size prop 1`] = `
+<button type="button" class="van-button van-button--default van-button--normal van-button--loading">
+  <div class="van-button__content">
+    <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 class="van-loading__circular" viewBox="25 25 50 50"><circle cx="50" cy="50" r="20" fill="none"></circle></svg></span>
+      <!---->
+    </div>
+    <!---->
+    <!---->
+  </div>
+</button>
+`;

src/button/test/index.spec.js

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+import { mount } from '../../../test';
+import Button from '..';
+
+test('loading-size prop', () => {
+  const wrapper = mount(Button, {
+    propsData: {
+      loading: true,
+      loadingSize: '10px',
+    },
+  });
+  expect(wrapper.html()).toMatchSnapshot();
+});