test: fix flaky TestAdd_Close_concurrent #3983

Job	Run time
testing_mac_os (1.20, macos-13, brew)	5m 13s
testing_mac_os (1.20, macos-13, brew)	5m 13s
run-tests-ce (1.20, 1.10, false, false)	4m 11s
run-tests-ce (1.20, 1.10, false, false)	4m 11s
testing_mac_os (1.20, macos-13, 1.10.15)	6m 13s
testing_mac_os (1.20, macos-13, 1.10.15)	6m 13s
run-tests-ce (1.20, 2.8, false, false)	4m 14s
run-tests-ce (1.20, 2.8, false, false)	4m 14s
testing_mac_os (1.20, macos-14, brew)	3m 58s
testing_mac_os (1.20, macos-14, brew)	3m 58s
run-tests-ce (1.20, 2.10, false, false)	4m 32s
run-tests-ce (1.20, 2.10, false, false)	4m 32s
testing_mac_os (stable, macos-13, brew)	6m 44s
testing_mac_os (stable, macos-13, brew)	6m 44s
run-tests-ce (1.20, master, false, false)	10m 32s
run-tests-ce (1.20, master, false, false)	5m 2s
testing_mac_os (stable, macos-13, 1.10.15)	6m 22s
testing_mac_os (stable, macos-13, 1.10.15)	6m 22s
run-tests-ce (stable, 1.10, false, false)	4m 24s
run-tests-ce (stable, 1.10, false, false)	4m 24s
testing_mac_os (stable, macos-14, brew)	4m 24s
testing_mac_os (stable, macos-14, brew)	4m 24s
run-tests-ce (stable, 2.8, false, false)	4m 26s
run-tests-ce (stable, 2.8, false, false)	4m 26s
run-tests-ce (stable, 2.10, false, false)	4m 43s
run-tests-ce (stable, 2.10, false, false)	4m 43s
run-tests-ce (stable, master, false, false)	10m 12s
run-tests-ce (stable, master, false, false)	5m 15s
run-tests-ce (master, true, 1.20)	14m 5s
run-tests-ce (master, true, 1.20)	14m 5s
run-tests-ce (master, true, 1.20, false)	14m 49s
run-tests-ce (master, true, 1.20, false)	14m 49s
	3h 27m 37s

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