test: create /tmp/cephtest/mnt.{id}

The workunit task assumes that a mount exists
at /tmp/cephtest/mnt.{id}
This patch creates the path if it doesn't
exist, enabling workunits to run in the absense
of kclient or ceph-fuse tasks.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by:  Sam Lang <sam.lang@inktank.com>