Add automated test for "Live email notifications with filter"

The manual test is here: Live email notifications with filter

Existing automated coverage

• LiveNotificationEmailEventFilterTest#isEventFiltered: Unit-level only: the live email event filter consults the filter manager. No mail is sent.
• NotificationsSettingsIT#customFiltersAndLiveData: Only the UI creation of custom filters (including one on the Email channel), not their effect.

Missing automated coverage

• Start the instance with notifications.emails.live.graceTime=0 and GreenMail; create pages a/b/c/d/e.
• User U1 (with an email address) follows the entire wiki, sets "Email Frequency" to Live and turns on all Email toggles.
• As U1, add an Ignore filter on page c (page and children), event type "A page is modified", channel Email.
• As another user, update pages a, b, c, d and e.
• Assert that U1 receives live emails only for the updates of a and b, and none for c, d and e.

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

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