[3.12] gh-109974: Fix threading lock_tests race conditions (#110057) (#110346)

* gh-109974: Fix threading lock_tests race conditions (#110057)

Fix race conditions in test_threading lock tests. Wait until a
condition is met rather than using time.sleep() with a hardcoded
number of seconds.

* Replace sleeping loops with support.sleeping_retry() which raises
  an exception on timeout.
* Add wait_threads_blocked(nthread) which computes a sleep depending
  on the number of threads. Remove _wait() function.
* test_set_and_clear(): use a way longer Event.wait() timeout.
* BarrierTests.test_repr(): wait until the 2 threads are waiting for
  the barrier. Use a way longer timeout for Barrier.wait() timeout.
* test_thread_leak() no longer needs to count
  len(threading.enumerate()): Bunch uses
  threading_helper.wait_threads_exit() internally which does it in
  wait_for_finished().
* Add BaseLockTests.wait_phase() which implements a timeout.
  test_reacquire() and test_recursion_count() use wait_phase().

(cherry picked from commit 4e356ad)

* gh-109974: Fix more threading lock_tests race conditions (#110089)

* Add context manager on Bunch class.
* Bunch now catchs exceptions on executed functions and re-raise them
  at __exit__() as an ExceptionGroup.
* Rewrite BarrierProxy.test_default_timeout(). Use a single thread.
  Only check that barrier.wait() blocks for at least default timeout
  seconds.
* test_with(): inline _with() function.

(cherry picked from commit 743e357)

Author: vstinner