Add automated test for "Mention an user from a subwiki"

The manual test is here: Mention an user from a subwiki

Existing automated coverage

• MentionsIT#documentBody: Main wiki only: a global user is mentioned through a REST-saved mention macro and gets a tray notification.
• QuickActionsIT#mention: The CKEditor "@" autocomplete inserts a mention of a main-wiki user. No notification is checked.
• MentionPageTest#mentionNotificationTemplateOtherWiki: Unit-level: the mention notification template shows the wiki of a mention made on another wiki.

Missing automated coverage

• Create a subwiki and a local user UL1 on it with an email address; mail is configured on the main wiki only and GreenMail is used.
• As UL1, turn on the Mentions alert and email toggles and turn the Pages toggles off.
• As Admin, edit a subwiki page in CKEditor, type "@", pick UL1 from the dropdown and save.
• Trigger the subwiki "Notifications daily email" job.
• As UL1, assert that the mention notification is in the tray.
• Assert that UL1 received the email with the mention (sent with the main-wiki mail configuration, see XWIKI-12435).

Suggested location: xwiki-platform-core/xwiki-platform-mentions/xwiki-platform-mentions-test/xwiki-platform-mentions-test-docker/src/test/it/org/xwiki/mentions/test/ui/MentionsIT.java (as a new @WikisSource test)

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