test: add test case for Q22 (webfansplz#551)

Author: holazz

questions/22-custom-element/README.md

@@ -0,0 +1,13 @@
+import { mount } from "@vue/test-utils"
+import { describe, it, expect } from "vitest"
+
+import App from "./App.vue"
+
+describe("custom-component", () => {
+  it("should work", () => {
+    const wrapper = mount(App, { attachTo: document.body })
+    const custom = wrapper.find('vue-js')
+    expect(custom.exists()).toBeTruthy()
+    expect(custom.element.shadowRoot?.innerHTML).toBe("<span>Hello Vue.js</span>")
+  })
+})