Add automated test for "Add exclusive filters" (User Mentions Tests)

The manual test is here: Add exclusive filters

Existing automated coverage

• MentionsIT#documentBody: Only the base case: a mention in page content gives the mentioned user a tray notification. No filter and no email.
• ScopeNotificationFilterTest#targetableEvent: Unit-level handling of targetable events (such as mentions) by scope filters.

Missing automated coverage

• Create pages a/b/c/d/e; U1 has an email address, picks the Daily email frequency, turns on the Mentions alert and email toggles and turns the Pages toggles off.
• As U1, add an Ignore filter on page c (page and children), event type Mentions, channel Alert.
• As U1, add an Ignore filter on page d (page and children), event type Mentions, channel Email.
• As Admin, mention U1 on each of a, b, c, d and e (in the WYSIWYG editor).
• Trigger the "Notifications daily email" job.
• Assert that U1's tray shows mentions only for a and b.
• Assert that the email received by GreenMail lists mentions only for a, b and c.

Suggested location: xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-test/xwiki-platform-mentions-test-docker/src/test/it/org/xwiki/mentions/test/ui/MentionsIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).