Fixed #23682 -- Enhanced circular redirects detection in tests.

Please see [#23682](https://code.djangoproject.com/ticket/23682).
Thanks for the better wording. I've changed "the" to "a", as this is a new value and it's somewhat arbitrary -- an early version of the HTTP standard required a maximum of 5 redirects ([RFC 2068 section 10.3](http://tools.ietf.org/html/rfc2068#section-10.3)) and some versions of IE allow only 10. Does this sound good?