Improve tests performance by moving /tmp on tmpfs

Before doing this, we'll want to characterize the state of /tmp utilization on the current slaves and also reevaluate swap allocation (I generally recommend increasing swap space by the size of any new tmpfs filesystems as a safety net against OOM conditions from excess data written there).

I think a better approach may be to run testr under eatmydata. Then we don't have to maintain state on the slaves themselves, instead individual projects can make use of the less reliable but speedier IO. This may not give us all of the speedup that a tmpfs would, but it should be much easier to manage.