bdev/raid: Print a message indicating that RAID tests were skipped

RAID tests are skipped when the OS is not Linux or if nbd isn't available.

Change-Id: I141a01603f849d38a1772686a0747beb08f7c265
Signed-off-by: WangHaiLiang <hailiangx.e.wang@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/451723
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-by: Liang Yan <liang.z.yan@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

test/bdev/bdev_raid.sh

@@ -98,6 +98,8 @@ function raid_function_test() {
 		fi
 
 		killprocess $raid_pid
+	else
+		echo "skipping bdev raid tests."
 	fi
 
 	return 0