Fixed flakiness in ContainerLoggerTest.DefaultToSandbox.
Fixed flakiness in ContainerLoggerTest.DefaultToSandbox.

The test needs to wait for the task to finish before examining
the resulting files.

Review: https://reviews.apache.org/r/42842/

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