csdk: fixed compiler warnings in stacktests.cpp

csdk: fixed compiler warnings in stacktests.cpp

-Wsign-compare fixed using the `u` suffix on hardcoded
values so the compiler knows they are unsigned values.

Change-Id: Ic5261e816e477e90f089ddc468606b710115bded
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/17715
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Way Vadhanasin <wayvad@microsoft.com>
Reviewed-by: Rick Bell <richard.s.bell@intel.com>