gh-103758: Add test for multiple os.register_at_fork registrations

[plfj]

Fixes gh-103758.
Supersedes and closes gh-103759.

Summary

Adds a regression test in Lib/test/test_os/test_posix.py to verify that registering callbacks multiple times via os.register_at_fork() correctly preserves both old and newly registered handlers across successive forks.

Specifically, it verifies:

• before handlers execute in LIFO (reverse) order.
• after_in_parent handlers execute in FIFO order.
• after_in_child handlers execute in FIFO order.
• Handlers registered after an earlier fork accumulate and run alongside previous generations in subsequent forks.

Note on Previous Work

This issue was originally reported by @itsankitkp with an initial implementation in #103759.

This PR supersedes #103759 by addressing reliability issues in the earlier attempt:

• Eliminates multi-process race conditions on a shared pipe by validating events per-process in memory.
• Eliminates pipe token-starvation deadlocks on child exit.
• Uses test.support.wait_process to reap child processes cleanly and verify exit status.

Credit is attributed to the original author via the commit trailer (Co-authored-by).

Checklist

• Tested locally with ./python -m test -v test_os.test_posix
• All required CI checks passed cleanly
• No NEWS entry required (test-only change as per Devguide)

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