LU-10026 tests: Fix sanity test_56ab for CSDC

LU-10026 tests: Fix sanity test_56ab for CSDC

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

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

Signed-off-by: Wei Liu <sarah@whamcloud.com>
Change-Id: Ib0108f4507a7b46ad5ca973ef43351e005232edf
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52572
Reviewed-by: Colin Faber <cfaber@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>