Add jest for unit tests

tests/App.test.js

@@ -0,0 +1,12 @@
+import React from "react";
+import renderer from "react-test-renderer";
+
+import App from "./App";
+
+
+describe("<App />", () => {
+    it("has 1 child", () => {
+        const tree = renderer.create(<App />).toJSON();
+        expect(tree.children.length).toBe(1);
+    });
+});