Fixes the flaky AllocatorTest/ResourcesUnused test.

From: Thomas Marshall <twm378@gmail.com>
Review: https://reviews.apache.org/r/9996

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1459117 13f79535-47bb-0310-9956-ffa450edef68
Fixes the flaky AllocatorTests.TaskFinished unit test.

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

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1459118 13f79535-47bb-0310-9956-ffa450edef68