[3.11] gh-109974: Fix threading lock_tests race conditions (GH-110057)

[miss-islington]

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)

Co-authored-by: Victor Stinner vstinner@python.org

• Issue: test_threading: test_set_and_clear() failed on Windows x64 #109974

[vstinner]

Oh, tests fail with:

AttributeError: module 'test.support' has no attribute 'sleeping_retry'

I will wait until changes are backported to 3.12 to backport these changes properly to 3.11.