Add automated test for "Notifications Custom Filters Preferences"

The manual test is here: Notifications Custom Filters Preferences (Since 13.2 RC1)

Existing automated coverage

• NotificationsSettingsIT#filterAndWatchedPage: Custom filters in the user Notification settings (rendered by the same macro in XWikiUserNotificationsSheet): created via watch, changed, deleted, persisted
• NotificationsSettingsIT#customFiltersAndLiveData: Adds custom filters through the +Add filter modal in administration and user settings and checks the live data

Missing automated coverage

• Create custom filters in the user Notification settings (+Add filter)
• Create a page with the notificationsCustomFiltersPreferences macro (inserted via CKEditor Insert > Other Macros)
• Assert the macro on the page lists the same filters as the Notification settings
• Add a filter and change an existing one in Notification settings; assert the page macro reflects it
• Delete a filter and change another from the page macro; assert Notification settings reflect it

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/NotificationsSettingsIT.java

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