fix: add button-group test

packages/button-group/__tests__/ButtonGroup.spec.js

@@ -0,0 +1,16 @@
+import ButtonGroup from '../ButtonGroup.vue'
+import { mount } from '@vue/test-utils'
+
+describe('ButtonGroup.vue', () => {
+  it('should show content', () => {
+    const content = 'foo'
+
+    const wrapper = mount(ButtonGroup, {
+      slots: {
+        default: content
+      }
+    })
+
+    expect(wrapper.text()).toContain(content)
+  })
+})