Add security integration tests

[lezzago]

Is your feature request related to a problem? Please describe.
Since the notification plugin is going to support integration with security, please add integration tests that test security permission scenarios.
This should include make sure if user has permission to access a resource or not and checking for filterby backend roles works (reference).

You can refer to the Alerting plugin to see how security integration tests were done:
Security tests for destinations
Security tests for monitors

[lezzago]

Please note this is an enhancement on the existing integration tests where it tests the security scenarios with the plugin to ensure only users with the correct permissions can access certain resources or APIs.

[lezzago]

These tests are to check basic security use cases like a user must get permission denied error when trying to access a Notification API that they do not have cluster permissions for. Including these tests will help reduce manual testing for security cases.