fix: replace fixed sleeps with polling in flaky TestChildProcessCleanup tests #740

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	31s
checks / test (3.10, locked, windows-latest)	2m 24s
checks / test (3.10, locked, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.14, lowest-direct, windows-latest)	2m 35s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.14, locked, windows-latest)	3m 1s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.13, lowest-direct, windows-latest)	2m 39s
checks / test (3.11, lowest-direct, ubuntu-latest)	55s
checks / test (3.11, locked, ubuntu-latest)	53s
checks / test (3.12, lowest-direct, windows-latest)	3m 34s
checks / test (3.13, locked, windows-latest)	2m 45s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.11, locked, windows-latest)	2m 26s
checks / test (3.11, lowest-direct, windows-latest)	3m 4s
checks / test (3.12, lowest-direct, ubuntu-latest)	55s
checks / test (3.13, lowest-direct, ubuntu-latest)	54s
checks / test (3.10, lowest-direct, windows-latest)	2m 25s
checks / test (3.14, locked, ubuntu-latest)	2m 16s
checks / test (3.12, locked, windows-latest)	3m 42s
all-green	2s
	40m 2s

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