[IoT-1562] Handle unit test segfaults at notification service

[IoT-1562] Handle unit test segfaults at notification service

Fix unittest segfault caused by discovery failure.
It is because external consumers or providers on the same network
interupt discovery or subscription precedure.
The test will be terminated once this situation happens,
even though precondition of the unittest is guided
to prevent this issue.

Change-Id: I0f763db760d8d2af0a185b9e7c3c6e71dfc5e5bf
Signed-off-by: cc <ch79.cho@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14851
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: JungYong KIM <jyong2.kim@samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>
(cherry picked from commit eb8ec52d48bb8def2a8862118d2fb97aed4c3c2f)
Reviewed-on: https://gerrit.iotivity.org/gerrit/14987
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>