[IOT-1592] Fix unit test of notification service.

[IOT-1592] Fix unit test of notification service.

Dependency of hippomocks is removed in notification service

Bug: https://jira.iotivity.org/browse/IOT-1562
Change-Id: I4d8b44b1c06c1abcc73a15fa87db94056a642b9d
Signed-off-by: KIM JungYong <jyong2.kim@samsung.com>
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/16377
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>