Add tests for /index.html

[TicTocLuke]

Would be handy to test home pages (index.html at the root of the site).

I noticed Vercel throws a ERR_TOO_MANY_REDIRECTS error in this case when cleanUrls=true. I'll raise the issue with Vercel

Update: The Vercel issue appears to be a misconfiguration on our end with Cloudfront, but still think this would be useful :)

[slorber]

Agree, but not sure there's any host that has different behavior for /index.html that is different from /file/index.html. That's worth being sure of that though.