Improve robustness of subshell concurrency tests #2617

Job	Run time
build (ubuntu-latest, 3.9)	3m 2s
check_release	1m 33s
link_check	3m 31s
build (ubuntu-latest, 3.13)	3m 46s
Make SDist	23s
build (windows-latest, 3.9)	6m 23s
test_docs	47s
build (windows-latest, 3.13)	6m 23s
Test Lint	1m 6s
Test Minimum Versions	2m 22s
build (macos-latest, 3.9)	3m 20s
build (macos-latest, 3.13)	5m 1s
Test Prereleases	2m 6s
test_without_debugpy (ubuntu-latest, 3.9)	2m 5s
build (ubuntu-latest, pypy-3.9)	3m 6s
build (macos-latest, 3.10)	3m 30s
build (ubuntu-latest, 3.11)	2m 59s
build (ubuntu-latest, 3.12)	3m 24s
Install from SDist and Test	2m 2s
coverage	1s
tests_check	8s
	56m 58s

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