fix: replace fixed sleeps with polling in flaky TestChildProcessCleanup tests #739

Job	Run time
checks / pre-commit	36s
checks / readme-snippets	11s
checks / test (3.10, locked, windows-latest)	2m 13s
checks / test (3.11, lowest-direct, windows-latest)	3m 1s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.11, locked, ubuntu-latest)	44s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.11, locked, windows-latest)	2m 25s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 8s
checks / test (3.10, locked, ubuntu-latest)	47s
checks / test (3.12, locked, windows-latest)	3m 17s
checks / test (3.13, lowest-direct, windows-latest)	2m 24s
checks / test (3.12, lowest-direct, windows-latest)	3m 27s
checks / test (3.14, lowest-direct, windows-latest)	2m 24s
checks / test (3.13, lowest-direct, ubuntu-latest)	49s
checks / test (3.11, lowest-direct, ubuntu-latest)	48s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.13, locked, ubuntu-latest)	43s
checks / test (3.13, locked, windows-latest)	2m 5s
checks / test (3.10, lowest-direct, windows-latest)	3m 3s
checks / test (3.14, locked, windows-latest)	3m 1s
checks / test (3.14, locked, ubuntu-latest)	2m 16s
all-green	4s
	38m 5s

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