Add automated test for "Load older notifications"

The manual test is here: Load older notifications

Existing automated coverage

• NotificationsIT#simpleNotifications: With more than 10 notifications, the tray shows 10 by default (10 unread, 0 read).
• InternalHtmlNotificationRendererTest#renderMany: Unit-level: the "load more" placeholder div is rendered when the batch is full.

Missing automated coverage

• With more than 10 notifications for the user (for example 15), open the bell and assert that 10 are shown.
• Click "Load older notifications" (needs a page-object method on NotificationsTrayPage or NotificationsContainerElement).
• Assert that the list grows by the next batch (15 here; with more than 20, it becomes 20), with older events and no duplicates.
• Assert that the button disappears once all notifications are loaded.

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).