Update test to fail on unset config.base

`pnpm test:workaround`, which deletes config.base, will now timeout.

This is in response to:

- vitest-dev/vitest#4686 (comment)

demo.test.js

@@ -1,3 +1,11 @@
-import { test } from 'vitest'
+import { expect, test } from 'vitest'
 
-test('OK in <=1.0.0-beta.4+vite <=5.0.0-beta.7, breaks otherwise', () => null)
+test('OK in <=1.0.0-beta.4+vite <=5.0.0-beta.7, breaks otherwise', async () => {
+  // This will timeout when config.base is undefined.
+  var page = window.open('/foobar/hello.html')
+  await new Promise(resolve => page.addEventListener('load', resolve))
+
+  let e = page.document.querySelector('body p')
+
+  expect(e.textContent).toContain('Hello, World!')
+})