Fixed flaky MasterTest.OrphanTasksMultipleAgents test.
Fixed flaky MasterTest.OrphanTasksMultipleAgents test.

See the attached bug for details.

Review: https://reviews.apache.org/r/53318

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