BUG Fixing Director::test() failing on BASE_URL prefixed URLs

Example: you have a site in a sub-directory off the webroot, you call
->Link() on a SiteTree record, which returns "/[sitedir]/my-page", and
you pass this URL to Director::test(). It's a valid URL, but
Director::test() will throw a 404.

Director::test() should be ensuring that all URLs passed to it are
properly made relative, not just in the case where it thinks the URL
is absolute.

This will fix an issue where ErrorPageTest fails when running tests
on a site in a sub-directory off a shared webroot, e.g. http://localhost/mysite.