[META] Add test coverage for Group Notify

Problem/Motivation

There are no automated tests for this module.

Proposed resolution

Add them. ;) We can use child issues for specific things to test.

Remaining tasks

1. Figure out everything we want to test.
2. Open child issues for each.
3. Once we have any working tests, configure this project node to run tests automatically on commits and patches posted in issues. See #3557728: Add GitLab CI pipelines and get all jobs passing