Rewrite TestChildProcessCleanup with socket-based deterministic liven… #1428

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	33s
checks / test (3.11, lowest-direct, windows-latest)	3m 30s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 1s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.12, lowest-direct, windows-latest)	3m 46s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, windows-latest)	2m 27s
checks / test (3.11, locked, windows-latest)	2m 31s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, locked, windows-latest)	2m 22s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	5m 22s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.13, locked, windows-latest)	2m 42s
checks / test (3.14, locked, windows-latest)	3m 25s
checks / test (3.14, lowest-direct, windows-latest)	2m 21s
checks / test (3.14, locked, ubuntu-latest)	2m 16s
checks / test (3.13, lowest-direct, ubuntu-latest)	55s
checks / test (3.13, lowest-direct, windows-latest)	2m 50s
all-green	5s
	42m 53s

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