Add automated test for "Add an image with caption"

The manual test is here: Add an image with caption

Existing automated coverage

• ImageIT#imageWithCaption: Image dialog (Tree tab), tick Caption, Insert, Save & View; asserts the captioned image source.
• ImageIT#imageWithLinkAndCaptionUI: Captioned image wrapped in a link; caption edited in-line in the figcaption; asserts the saved source.
• ImageIT#pasteCaptionedImage: Pasted captioned images (figure/figcaption) round-trip through the editor.
• RealtimeWYSIWYGEditorIT#imageWithCaption: Upload tab of the image dialog, tick Caption, Insert, caption edited in-line; asserts the upload and the saved source (realtime editor).
• image12.test: Parsing of the captioned image syntax with a nested link in the caption (figure/figcaption events).

Missing automated coverage

• Edit the page in CKEditor Source mode, enter a captioned image whose caption is a link to https://www.xwiki.org (the syntax given in the manual test), switch back to WYSIWYG; assert a figure whose figcaption contains the link is shown.
• Modify the caption text in-line below the image and assert the source keeps the link inside the updated caption.
• Save & View and assert the view page renders a figure with the image and a figcaption containing the updated text and the xwiki.org link (both for the Source-mode image and the Upload-tab captioned image).

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/ImageIT.java

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