gh-98458: unittest: bugfix for infinite loop while handling chained exceptions that...

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