Add automated test for "Check the text version of an email notification"

The manual test is here: Check the text version of an email notification

Existing automated coverage

• NotificationsEmailsIT#notificationsEmails: The user follows the wiki and turns on the create event for email; another user creates 2 pages; the daily email job is triggered and the text/plain part must contain expectedPlainTextMail1/2.txt, which hold the "[create]" label of each event.
• DefaultNotificationEmailRendererTest#renderPlainText: Unit-level: the plain-text renderer is called (mocked templates).

Missing automated coverage

• Have U1 follow the entire wiki, pick the Daily frequency with "Details of the changes" = Standard, and turn on all email toggles (Pages, Comments, Mentions, Likes).
• As Admin, update a page, comment on a page, delete a page, mention U1 in a page and like a page (in addition to the create already covered).
• Trigger the "Notifications daily email" job.
• Assert that the text/plain part, under "Event(s) on the wiki:", lists every event performed by Admin.
• Assert that each event has the label of its type: [update], [addComment], [delete], the mention and the like events.

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

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