ARROW-13332: [C++] TSAN failure in TestAsyncUtil.ReadaheadFailed
ARROW-13332: [C++] TSAN failure in TestAsyncUtil.ReadaheadFailed

Minor cleanup fix from my earlier fix of this test in #10602

Closes #10719 from westonpace/bugfix/ARROW-13332--c-tsan-failure-in-testasyncutil-readaheadfaile

Authored-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>
Signed-off-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>

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