LU-8668 tests: print more information about dd failure

Don't drop the dd output so that we can see why this is failing.

Test-Parameters: trivial testlist=sanityn,sanityn,sanityn,sanityn
Test-Parameters: trivial testlist=sanityn,sanityn,sanityn,sanityn
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I3364bca75817ee4d0128079744e4b01a3ac938a0
Reviewed-on: http://review.whamcloud.com/22990
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>