Fixed flaky test GroupTest.GroupWatchWithSessionExpiration.
Fixed flaky test GroupTest.GroupWatchWithSessionExpiration.

This test became flaky due to a recent change in MESOS-935 that made
Group update watchers in expired(), this introduced a race condition
on the 'membership.get().cancelled()' Future (Group thread sets it
and the test thread reads it).

Review: https://reviews.apache.org/r/17714

abudnik/mesos,gsantovena/mesos,gsantovena/mesos,neilconway/mesos,abudnik/mesos,verizonlabs/mesos,gsantovena/mesos,chhsia0/mesos,zmalik/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,andschwa/mesos,craimbert/mesos,dforsyth/mesos,kaysoky/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,neilconway/mesos,craimbert/mesos,jpeach/mesos,neilconway/mesos,yuquanshan/customizedMesos,craimbert/mesos,asamerh4/mesos,Gilbert88/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,asamerh4/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,jpeach/mesos,dforsyth/mesos,zmalik/mesos,craimbert/mesos,dforsyth/mesos,chhsia0/mesos,craimbert/mesos,zmalik/mesos,jpeach/mesos,verizonlabs/mesos,neilconway/mesos,Gilbert88/mesos,zmalik/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,reneploetz/mesos,andschwa/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,abudnik/mesos,jpeach/mesos,abudnik/mesos,andschwa/mesos,reneploetz/mesos,zmalik/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,gsantovena/mesos,asamerh4/mesos,abudnik/mesos,craimbert/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,asamerh4/mesos,kaysoky/mesos,andschwa/mesos,verizonlabs/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,neilconway/mesos,kaysoky/mesos,kaysoky/mesos,Gilbert88/mesos,gsantovena/mesos,chhsia0/mesos,verizonlabs/mesos,andschwa/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,zmalik/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,kaysoky/mesos,asamerh4/mesos,neilconway/mesos