Fixed flakyness in 'SlaveTest.ExecutorShutdownGracePeriod'.
Fixed flakyness in 'SlaveTest.ExecutorShutdownGracePeriod'.

On slow systems, it could happen that an additional offer was
delivered to the scheduler before the test case finished.

Review: https://reviews.apache.org/r/63113/

abudnik/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,zmalik/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,andschwa/mesos,kaysoky/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,asamerh4/mesos,andschwa/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,andschwa/mesos,zmalik/mesos,Gilbert88/mesos,zmalik/mesos,dforsyth/mesos,asamerh4/mesos,gsantovena/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,zmalik/mesos,abudnik/mesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,zmalik/mesos,jpeach/mesos,dforsyth/mesos,asamerh4/mesos,gsantovena/mesos,chhsia0/mesos,asamerh4/mesos,kaysoky/mesos,jpeach/mesos,asamerh4/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,zmalik/mesos,Gilbert88/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,zmalik/mesos,Gilbert88/mesos,andschwa/mesos