Replace enzyme with testing-library/react

Enzyme is not compatible with react 17.

Change-Id: If9bca2c482bfe10a18d2ee2bc213dab966849b5b

src/App.test.js

@@ -1,9 +1,9 @@
 import React from "react";
-import { shallow } from "enzyme";
+import { render } from "@testing-library/react";
 import App from "./App";
 
 describe("App", () => {
   it("renders", () => {
-    shallow(<App />);
+    render(<App />);
   });
 });