Fixed test `MasterTestPrePostReservationRefinement.LaunchGroup`.
Fixed test `MasterTestPrePostReservationRefinement.LaunchGroup`.

Added a missing scheduler acknowledgment call so that both
expected task status updates can be delivered (instead of just one).

Review: https://reviews.apache.org/r/69178/

kaysoky/mesos,gsantovena/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,jpeach/mesos,Gilbert88/mesos,gsantovena/mesos,gsantovena/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,chhsia0/mesos,gsantovena/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,Gilbert88/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,reneploetz/mesos,gsantovena/mesos,abudnik/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos