Fix scons build issue

Fix scons build issue

The scons fails due to 'stacktest_env' is not defined.
Replace stacktest_env with unittests_env.

Change-Id: I9ec68a05f7c576be3686fe12f93077cd6c7ac7ca
Signed-off-by: Sakthivel Samidurai <sakthivel.samidurai@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/412
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sashi Penta <sashi.kumar.penta@intel.com>
Reviewed-by: Ossama Othman <ossama.othman@intel.com>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>