Add automated test for "Notifications count (below 21)"

The manual test is here: Notifications count (below 21)

Existing automated coverage

• NotificationsIT#simpleNotifications: Checks the badge value ("20+", then 20 after one is marked read, then 1) against the number of generated events, while the list shows only the first 10.
• MentionsIT#documentBody: With a single notification, the badge (1) matches the number of unread entries in the list (1).

Missing automated coverage

• Produce between 11 and 20 unread notifications for U1 and read the badge count.
• Open the bell and click "Load older notifications" until it disappears.
• Assert that the number of listed (unread) notifications equals the badge count.

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/NotificationsIT.java

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