Add automated test for "Rename or move an extension page"

The manual test is here: Rename or move an extension page

Existing automated coverage

• DocumentsDeletingListenerTest#onEvent: Unit test: when a refactoring job (delete, or the source deletion of a move/rename) touches pages of an installed extension, an ExtensionBreakingQuestion is asked and only the selected pages are processed.
• ExtensionBreakingQuestionTest#get: Unit test of the question model (pages grouped by extension, selection).
• RenamePageIT#renamePageWithUsedClass: Same job question UI (JobQuestionPane: tree selection, confirm, cancel) during a rename, but for the "used XClass" question, not the extension one.

Missing automated coverage

• Go to a page of an installed XAR extension (e.g. Help/Applications/Contributors/Charlie Chaplin), open More Actions > Move / Rename, set a new title, click Rename
• Assert the question "You are about to rename pages that belong to extensions." is displayed (JobQuestionPane#getQuestionTitle)
• Select the page in the question tree, click Confirm, and assert the rename is done (new page exists, old one moved)
• Also check that cancelling the question leaves the page in place

Suggested location: xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-test/xwiki-platform-flamingo-skin-test-docker/src/test/it/org/xwiki/flamingo/test/docker/RenamePageIT.java

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