Add automated test for "Notifications Email Preferences Macro"

The manual test is here: Notifications Email Preferences Macro

Existing automated coverage

• NotificationsSettingsIT#globalAndOtherUserSettings: Reads and changes Details of the changes (email diff type) in user settings, rendered by the notificationsEmailPreferences macro, and asserts persistence

Missing automated coverage

• Add page-object support for the Email Frequency (interval) setting
• Create a page with the notificationsEmailPreferences macro (inserted via CKEditor Insert > Other Macros)
• Assert Email Frequency and Details of the changes on the page match the user Notification settings
• Change both in Notification settings; assert the page macro shows the new values
• Change both from the page macro; assert Notification settings show them

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).