`test_threading` failing randomly on CI

See https://github.com/python/cpython/actions/runs/12953185538/job/36134744339?pr=128869

test_reinit_tls_after_fork (test.test_threading.ThreadJoinOnShutdown.test_reinit_tls_after_fork) ... Warning -- Uncaught thread exception: AssertionError
Exception in thread Thread-34 (do_fork_and_wait):
Traceback (most recent call last):
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/test_threading.py", line 1438, in do_fork_and_wait
    support.wait_process(pid, exitcode=50)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/support/__init__.py", line 2205, in wait_process
    raise AssertionError(f"process {pid} is still running "
                         f"after {dt:.1f} seconds")
AssertionError: process 10953 is still running after 300.2 seconds
ok

從關聯的 CI 執行和 Lib/test/test_threading.py 開始，重點查看 ThreadJoinOnShutdown.test_reinit_tls_after_fork 及其 do_fork_and_wait 輔助函式。追蹤對 Lib/test/support/init.py: wait_process 的呼叫，並重現子程序掛起 300 秒的問題。完成的標準是測試不再間歇性失敗，也不會讓子程序繼續在 CI 上執行。