chore(notification_center)[OASIS-5764]: Add all unit tests for the notification_center module

[alibaker-optimizely]

Summary

• The notification_center module did not have a unit test associated with it.
• This PR addresses that to ensure 100% coverage of this module.

Test plan

addNotificationListener

• only valid listener types should have listeners registered
• the listenerId should be returned upon successful registration
• cannot register the same callback twice

removeNotificationListener

• should return true if the listener is successfully removed
• should only remove the specified listener using the listenerId

clearAllNotificationListeners

• should remove all listeners of all types

clearNotificationListeners

• should remove only the specified notification type when there are no listeners registered
• should remove only the specified notification type and leave all other notification type listeners still registered

sendNotifications

• checks that each notification type listeners executes the callback with the passed in notification data from the user

Issues

• Addresses OASIS-5764

[coveralls]

Coverage increased (+0.1%) to 97.559% when pulling 31df225 on alibaker/add-notification-center-unit-tests into bc84698 on master.

[coveralls]

Coverage increased (+0.1%) to 97.559% when pulling 931dea2 on alibaker/add-notification-center-unit-tests into bc84698 on master.

[mjc1283]

Great work, I appreciate the thorough test coverage here. I have just some minor suggestions - overall this looks good to me.