Add automated test for "Add a link to an existing page, attachment and URL"

The manual test is here: Add a link to an existing page, attachment and URL

Existing automated coverage

• LinkIT#insertLinks: Link toolbar button, page resource chosen via the suggest input, then Attachment resource type chosen and an attachment selected; asserts the generated wiki source.
• LinkIT#createLinkToNewPageFromSelectedText: Opening the link dialog on selected text keeps the selection as link label.
• LinkIT#createLinkAfterChangingResourceType: Selects the URL resource type in the link dialog (then cancels); no URL link is created.

Missing automated coverage

• Select a word, open the Link dialog, type http://google.ro, choose the URL resource type, submit; assert the source contains a url:http://google.ro link labelled with that word.
• Create the page and attachment links on selected words (not at the caret) and assert the selected words become the link labels.
• Save & View and assert the three rendered links point to the target page view URL, the attachment download URL and the external URL (optionally click the page link and assert the target page is displayed).

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).