Fix the directory index test

Related to #104
It should...but now that I'm looking at the code, I see @penibelst's point. This distinction between folder or file is a bit tricky. We don't ever actually make a distinction, I don't think, about which is missing. We just say that the whole path is invalid. So whether  `folder-php` is there or not, or whether `README.md` is there or not, a link to `folder-php/README.md` with one of those missing causes a failure.