[3.12] gh-109582: test_fork_signal_handling should wait for event (GH-109605) (#109695)

[3.12] gh-109582: test_fork_signal_handling should wait for event (GH-109605) (#109695)

gh-109582: test_fork_signal_handling should wait for event (GH-109605)

Sometimes the child_handled event was missing because either
the child quits before it gets a chance to handle the signal,
or the parent asserts before the event notification is
delivered via IPC.  Synchronize explicitly to avoid this.
(cherry picked from commit 608c1f3083ea1e06d383ef1a9878a9758903de4b)

Co-authored-by: Davide Rizzo <sorcio@gmail.com>