Fixes the flaky AllocatorTests.TaskFinished unit test.
Fixes the flaky AllocatorTests.TaskFinished unit test.

From: Thomas Marshall <twm378@gmail.com>
Review: https://reviews.apache.org/r/9887

git-svn-id: 03d7800a49d8905ef4fbcc296184319e19243493@1459118 13f79535-47bb-0310-9956-ffa450edef68

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