Improve robustness of subshell concurrency tests #2618

Job	Run time
Test Lint	54s
check_release	1m 37s
build (ubuntu-latest, 3.9)	3m 14s
link_check	1m 43s
build (ubuntu-latest, 3.13)	3m 23s
build (windows-latest, 3.9)	7m 18s
Make SDist	31s
test_docs	45s
build (windows-latest, 3.13)	7m 55s
build (macos-latest, 3.9)	3m 1s
Test Minimum Versions	2m 14s
Test Prereleases	1m 55s
build (macos-latest, 3.13)	5m 8s
test_without_debugpy (ubuntu-latest, 3.9)	1m 59s
build (ubuntu-latest, pypy-3.9)	3m 4s
build (macos-latest, 3.10)	3m 7s
build (ubuntu-latest, 3.11)	3m 1s
build (ubuntu-latest, 3.12)	3m 24s
Install from SDist and Test	1m 50s
coverage	12s
tests_check	5s
	56m 20s

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