Code review comment for lp:~cemil-azizoglu/mir/MesaBufferAllocatorTest-failure-under-sbuild

P.S. I did approve of the previous O_TMPFILE form, for the record. Only disapproved of unrelated build-dep changes being included (now fixed). Also, Cemil found there was a CI problem with it (like some CI machine didn't have a new enough kernel for O_TMPFILE?).