LU-8989 tests: fix sanity test_256() use of hardcoded filename

LU-8989 tests: fix sanity test_256() use of hardcoded filename

Description: test_256() in sanity test suite uses a hardcoded file name.
Switching to mktemp like other tests.

Test-Parameters: trivial
Change-Id: I013c86cea1af5769f81119aa2aaeb63766a2cec9
Signed-off-by: Chris Hanna <hannac@iu.edu>
Reviewed-on: https://review.whamcloud.com/27354
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>