Added a test to ensure correct headroom accounting.
Added a test to ensure correct headroom accounting.

This test verifies that the headroom accounting is correct in the
presence of subrole allocation. In particular, we need to ensure
that a subrole's allocation is accounted only once (not multiple
times in itself as well as all of its ancestors) when calculating
available the quota headroom.

Review: https://reviews.apache.org/r/70390

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