[3.11] [3.12] GH-110894: Call loop exception handler for exceptions in client_connected_cb (GH-111601) (GH-111632) (#111634)

* [3.12] GH-110894: Call loop exception handler for exceptions in client_connected_cb (GH-111601) (GH-111632)
(cherry picked from commit 9aa88290d82e2808eed84e7a63d0bf9623f84f53)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Call loop exception handler for exceptions in `client_connected_cb` of `asyncio.start_server` so that applications can handle it..
(cherry picked from commit 229f44d353c71185414a072017f46f125676bdd6)

* gh-111644: Fix asyncio test_unhandled_exceptions() (#111713)

Fix test_unhandled_exceptions() of test_asyncio.test_streams: break
explicitly a reference cycle.

Fix also StreamTests.tearDown(): the loop must not be closed
explicitly, but using set_event_loop() which takes care of shutting
down the executor with executor.shutdown(wait=True).
BaseEventLoop.close() calls executor.shutdown(wait=False).

(cherry picked from commit ac01e2243a1104b2154c0d1bdbc9f8d5b3ada778)

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Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Victor Stinner <vstinner@python.org>