[C++] Intermittent failure in TestNewScanner.Backpressure

For example:

https://github.com/ursacomputing/crossbow/actions/runs/3277989378
/jobs/5395881371#step:5:3133

**Reporter**: [Weston Pace](https://issues.apache.org/jira/browse/ARROW-18100) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-18100](https://issues.apache.org/jira/browse/ARROW-18100). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the TestNewScanner.Backpressure test and the linked Crossbow Actions failure at step 5. Reproduce the intermittent failure and inspect the test's scanner and backpressure behavior. Done means the failure is understood and the test no longer fails intermittently.

Written by the indexing model from the issue text.