EX-8281 tests: Fix sanity test_56ab when CSDC is enabled

EX-8281 tests: Fix sanity test_56ab when CSDC is enabled

Use /dev/urandom in sanity test_56ab so the data cannot be compressed

Lustre-change: https://review.whamcloud.com/52572
Lustre-commit: TBD  (from 1ce661dd56fb4b6ecc9e909805c6101bbd9c3161)

Test-Parameters: trivial testlist=sanity env=ONLY=56ab

Signed-off-by: Wei Liu <sarah@whamcloud.com>
Change-Id: I0ceb9afcbdc8443b5e04dff486e41621479dbd23
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52501
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Colin Faber <cfaber@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>