Add automated test for "Notifications Macro"

The manual test is here: Notifications Macro

Existing automated coverage

• NotificationsIT#displayNotificationsOnSubwikis: Page containing the notifications macro with useUserPreferences=false displays the expected events
• NotificationsIT#simpleNotifications: Events from another user shown in the notification tray (itself rendered by the notifications macro with useUserPreferences=true); mark as read in the tray persists after refresh

Missing automated coverage

• U1 has notifications on and watches the whole wiki; as Admin create, update and comment pages
• As U1 create a page with the notifications macro and useUserPreferences=true (inserted via CKEditor Insert > Other Macros)
• Assert the macro lists the same events as the notification tray
• Mark an event as read in the page macro and refresh
• Assert the event is shown as read in the macro and the tray unread count decreased

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).