Add automated test for "Link autocomplete"

The manual test is here: Link autocomplete

Existing automated coverage

• QuickActionsIT#link: Open-square-bracket link autocomplete with a page name (alice, a terminal page); asserts the link source.
• LinkIT#useLinkShortcutWhenTargetPageNameHasSpecialCharacters: Open-square-bracket link autocomplete to a terminal page with special characters; asserts the link source.

Missing automated coverage

• Create a nested page (Space.WebHome) and a terminal page, each with an attachment.
• Type an open square bracket followed by 2 characters of the nested page name, click the suggestion; assert a doc: link to the nested page WebHome.
• Type an open square bracket followed by 2 characters of each attachment name, click the suggestion; assert attach: links to both attachments.

Suggested location: xwiki-platform-core/xwiki-platform-ckeditor/xwiki-platform-ckeditor-test/xwiki-platform-ckeditor-test-docker/src/test/it/org/xwiki/ckeditor/test/ui/LinkIT.java

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