Added tests for the agent gone operation.
Added tests for the agent gone operation.

This change adds tests for marking an active agent as gone and also
another test for ensuring that running task on an agent marked as gone
are correctly transitioned to 'TASK_GONE_BY_OPERATOR'.

Review: https://reviews.apache.org/r/62480/

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