[C++][CI] CountingSemaphore sporadic test crash

This may be a fluke, but this crash appeared on CI:
https://github.com/apache/arrow/runs/4234285140?check_suite_focus=true#step:8:110

```Java

[==========] Running 134 tests from 23 test suites.
[----------] Global test environment set-up.
[----------] 7 tests from CancelTest
[ RUN ] CancelTest.StopBasics
[ OK ] CancelTest.StopBasics (0 ms)
[ RUN ] CancelTest.StopTokenCopy
[ OK ] CancelTest.StopTokenCopy (0 ms)
[ RUN ] CancelTest.RequestStopTwice
[ OK ] CancelTest.RequestStopTwice (0 ms)
[ RUN ] CancelTest.Unstoppable
[ OK ] CancelTest.Unstoppable (0 ms)
[ RUN ] CancelTest.SourceVanishes
[ OK ] CancelTest.SourceVanishes (0 ms)
[ RUN ] CancelTest.ThreadedPollSuccess
[ OK ] CancelTest.ThreadedPollSuccess (11 ms)
[ RUN ] CancelTest.ThreadedPollCancel
[ OK ] CancelTest.ThreadedPollCancel (11 ms)
[----------] 7 tests from CancelTest (23 ms total)

[----------] 2 tests from SignalCancelTest
[ RUN ] SignalCancelTest.Register
[ OK ] SignalCancelTest.Register (1 ms)
[ RUN ] SignalCancelTest.RegisterUnregister
[ OK ] SignalCancelTest.RegisterUnregister (111 ms)
[----------] 2 tests from SignalCancelTest (113 ms total)

[----------] 3 tests from CountingSemaphore
[ RUN ] CountingSemaphore.Basic

```

**Reporter**: [Antoine Pitrou](https://issues.apache.org/jira/browse/ARROW-14734) / @pitrou

_{**Note**: *This issue was originally created as [ARROW-14734](https://issues.apache.org/jira/browse/ARROW-14734). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the linked CI run and the CountingSemaphore.Basic test, then inspect the surrounding CountingSemaphore test implementation and its concurrency behavior. Reproduce the crash if possible and trace the failure to its cause; done means the sporadic CI crash is explained and the relevant test or implementation no longer fails.

Written by the indexing model from the issue text.