Add automated test for "Live email notifications on subwiki"

The manual test is here: Live email notifications on subwiki

Existing automated coverage

• PrefilteringLiveNotificationEmailSenderTest#testSendMail: Unit-level only: the live email sender builds and sends mails for prefiltered events (mocked mail sender).

Missing automated coverage

• Configure mail on the main wiki only, start GreenMail and set notifications.emails.live.graceTime=0.
• Create a subwiki and a local user on it with an email address.
• As that user, follow the entire subwiki, set "Email Frequency" to Live and turn on all Email toggles.
• As Admin, create, update and delete a page, comment on another page and mention the user in a page, all on the subwiki.
• Assert that the local user receives an email for each of these events (sent with the main-wiki mail configuration).

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/NotificationsEmailsIT.java (as a new @WikisSource test)

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