gh-98458: unittest: bugfix for infinite loop while handling chained exceptions that contain cycles (GH-98459)

* Bugfix addressing infinite loop while handling self-referencing chained exception in TestResult._clean_tracebacks()
* Bugfix extended to properly handle exception cycles in _clean_tracebacks. The "seen" set follows the approach used in the TracebackException class (thank you @iritkatriel for pointing it out)
* adds a test for a single chained exception that holds a self-loop in its __cause__ and __context__ attributes
(cherry picked from commit 72ec518203c3f3577a5e888b12f10bb49060e6c2)

Co-authored-by: AlexTate <0xalextate@gmail.com>