test(Button): add basic test case

src/button/test/index.spec.js

@@ -0,0 +1,12 @@
+import { mount } from '../../../test';
+import Button from '..';
+
+test('loading-size prop', () => {
+  const wrapper = mount(Button, {
+    propsData: {
+      loading: true,
+      loadingSize: '10px',
+    },
+  });
+  expect(wrapper.html()).toMatchSnapshot();
+});