Regression test portability: xargs

Linux xarg complains about an empty input, while NetBSD xargs does not.
This breaks tests where xargs is in a pipe after a command that should
be tested. Make sure we test the first command without xargs

BUG: 764655
Change-Id: I754d7d52332221c462ce3594f4e8d8d62ae606d5
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8280
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Anand Avati <avati@redhat.com>