{/* eslint-disable-next-line @next/next/no-sync-scripts */}
<script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script>
If you’re using a similar setup as I do - Github Pages + Next.js, you could paste this anywhere in the code outside the <Head> element in order for the test runner to pop up. The Script from next/script did not work for me, hence why I had to silence eslint.
Personally, I just placed it right after closing <Head>.
Note: The test runner doesn’t need to be there in the final solution. Your code is not checked at all, anyhow. I could complete the challenge with a basically empty Github Pages page.