Improve robustness of subshell concurrency tests using Barrier #2630

Job	Run time
check_release	1m 53s
build (ubuntu-latest, 3.9)	3m 6s
test_docs	41s
build (ubuntu-latest, 3.13)	3m 24s
Test Lint	57s
build (windows-latest, 3.9)	9m 55s
test_without_debugpy (ubuntu-latest, 3.9)	1m 55s
build (windows-latest, 3.13)	10m 35s
link_check	3m 34s
Make SDist	24s
build (macos-latest, 3.9)	3m 16s
Test Minimum Versions	2m 18s
build (macos-latest, 3.13)	4m 50s
Test Prereleases	1m 59s
build (ubuntu-latest, pypy-3.9)	4m 13s
build (macos-latest, 3.10)	3m 5s
build (ubuntu-latest, 3.11)	3m 2s
build (ubuntu-latest, 3.12)	3m 23s
Install from SDist and Test	1m 59s
coverage	7s
tests_check	4s
	1h 4m 40s

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