LU-17873 test: ignore WIFSIGNALED if rc is 0

LU-17873 test: ignore WIFSIGNALED if rc is 0

Ignored the checking resulst of WIFSIGNALED if the return status
of the "lctl test_create" thread is zero.

Test-Parameters: trivial envdefinitions=SLOW=yes,DEBUG_SIZE=64 mdtcount=1 testlist=mds-survey,mds-survey,mds-survey,mds-survey,mds-survey,mds-survey
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: Ifc3727d48010c9f00f38baff9ff91b5cc3afce5c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55194
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>