MINOR: [C++] Fix flaky fork safety test
MINOR: [C++] Fix flaky fork safety test

TestThreadPoolForkSafety.MultipleChildThreads could occasionally fail because of concurrent pushes to a vector.

Closes #12645 from pitrou/cpp-fork-safety-test

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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