Revert "remove unused file"

This reverts commit 8a825a6f.

src/react-app-env.d.ts

+/// <reference types="react-scripts" />

src/setupTests.ts

+// jest-dom adds custom jest matchers for asserting on DOM nodes.
+// allows you to do things like:
+// expect(element).toHaveTextContent(/react/i)
+// learn more: https://github.com/testing-library/jest-dom
+import "@testing-library/jest-dom";