gh-158336: Fix race in lock_tests test_set_and_clear

test_set_and_clear started N threads calling event.wait() and then slept
50 ms (wait_threads_blocked) before calling event.set() and event.clear().
If a thread only reached wait() after clear(), it saw the flag unset and
blocked for LONG_TIMEOUT, and Bunch.__exit__ gave up after SHORT_TIMEOUT.
This happened on CI in the TSan free-threading job.

Wait until all N threads are in the Condition's waiter list instead.
Condition.wait() appends the waiter while holding the lock, and
notify_all() releases every waiter in the list, so each of those threads
is guaranteed to be woken by set().

On a TSan free-threading debug build, pinned to 2 CPUs shared with 4
busy-looping processes, the test failed in 9 of 40 runs before this
change and 0 of 40 after.

Author: matthiasgoergens

Lib/test/lock_tests.py

@@ -572,8 +572,13 @@ def f():
 
         N = 5
         with Bunch(f, N):
-            # Threads blocked on event.wait()
-            wait_threads_blocked(N)
+            # Wait until all threads are registered as waiters in
+            # event.wait(). A thread that only reaches wait() after set()
+            # and clear() would block until the timeout, so a fixed sleep
+            # is not enough on a busy machine.
+            for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
+                if len(event._cond._waiters) >= N:
+                    break
 
             # Threads unblocked
             event.set()