tests: make EXPECT_WITHIN also fail on command failure

Change-Id: I051bbe2344f19e647826ee177932ae8b95747ebd
BUG: 764966
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4334
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>