LU-7884 utils: mkfs prevents the creation of files in /dev/shm

LU-7884 utils: mkfs prevents the creation of files in /dev/shm

When configuring llmount to use /dev/shm as its TMP directory
(for loopback files), mkfs.lustre complains that the loopback does
not exist and will not be created.
This patch fixes this by adding a "touch" instruction before any use
of mkfs.lustre in test-framework.sh

Signed-off-by: Quentin Bouget <quentin.bouget@cea.fr>
Change-Id: I1422d8af947db5ba5b262a9acd2239e99f0843c2
Reviewed-on: https://review.whamcloud.com/18984
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>