Adding test cases in a separate commit to make it easier to review how
the test results change after I98fbca8e.
* For mentions, the 'mentioned-users' extra parameter is copied to our
event (which is then used to avoid duplicate notifications).
* For user talk page edit, nothing special happens right now (we use
the target page title to avoid duplicate notifications, but this is
not apparent from the test case, since page titles are not present).
Bug: T281590
Bug: T253082
Change-Id: I153e7735f63f1e2643ed881281d807313cd699c3
