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.
Merge pull request #3424 from halkyon/director_test_relative

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