MRP-1194 tests: fix sanity test_27i to use test_mkdir()

Use test_mkdir() instead of mkdir.

Signed-off-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Change-Id: Ib58554906265b3acbcbaf05c107aa362389e1c25
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/3312
Reviewed-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>