Fixed #16816 -- Tweaked the test mock for `URLField.verify_exists` to allow...

Fixed #16816 -- Tweaked the test mock for `URLField.verify_exists` to allow tests to pass when there is no Internet connection available. Many thanks to Ramiro Morales, Aymeric Augustin and Florian Apolloner for the patch reviews.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17059 bcc190cf-cafb-0310-a4f2-bffc1f526a37