test: refactor test:demo script into demo/*/test.sh

demo/reactjs/src/App.test.tsx

@@ -0,0 +1,9 @@
+import { expect, test } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import { App } from './App';
+
+test('App rendered correctly', async () => {
+  render(<App/>)
+  const h2 = await screen.findByTestId('heading')
+  expect(h2.textContent).toEqual('Welcome to Liquid')
+})