Refactor unit test for Notification Error Handler

Problem: The current notification Error handler unit test uses 'systemOut' which on occasions causes the build to fail

Solution:

Use of log watchers as suggested in https://gerrit.onap.org/r/c/cps/+/129783/4/cps-service/src/test/groovy/org/onap/cps/notification/NotificationErrorHandlerSpec.groovy#31