[C++] Sporadic error in StressSourceGroupedSumStop

I just saw this occasional failure:
https://github.com/apache/arrow/actions/runs/3533672097/jobs/5929601817#step:11:294

```Java

[ RUN ] ExecPlanExecution.StressSourceGroupedSumStop
D:/a/arrow/arrow/cpp/src/arrow/compute/exec/plan_test.cc:850: Failure
Value of: _fut.Wait(::arrow::kDefaultAssertFinishesWaitSeconds)
Actual: false
Expected: true
Google Test trace:
D:/a/arrow/arrow/cpp/src/arrow/compute/exec/plan_test.cc:825: parallel
D:/a/arrow/arrow/cpp/src/arrow/compute/exec/plan_test.cc:822: unslowed
D:/a/arrow/arrow/cpp/src/arrow/compute/exec/exec_plan.cc:60: Plan was destroyed before finishing
```

**Reporter**: [Antoine Pitrou](https://issues.apache.org/jira/browse/ARROW-18398) / @pitrou

_{**Note**: *This issue was originally created as [ARROW-18398](https://issues.apache.org/jira/browse/ARROW-18398). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with cpp/src/arrow/compute/exec/plan_test.cc around lines 822-850 and review the linked CI failure for ExecPlanExecution.StressSourceGroupedSumStop. Inspect cpp/src/arrow/compute/exec/exec_plan.cc around line 60, then reproduce the intermittent failure. Done means the stress test reliably completes without the plan being destroyed before finishing.

Written by the indexing model from the issue text.