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

The workunit task assumes a mount exists at
/tmp/cephtest/mnt.{id}
This patch addes the path if the mount doesn't
exist. This allows workunits to run in the
absense of a kclient or cephfust task.

Signed-off-by: Joe Buck <jbbuck@gmail.com>