react-rust-postgres: create simple client

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-rust-postgres/frontend/src/App.test.js

@@ -0,0 +1,9 @@
+import React from 'react';
+import { render } from '@testing-library/react';
+import App from './App';
+
+test('renders learn react link', () => {
+  const { getByText } = render(<App />);
+  const linkElement = getByText(/learn react/i);
+  expect(linkElement).toBeInTheDocument();
+});