Add automated test for "Insert Info Macro in CKEditor"

The manual test is here: Insert Info Macro in CKEditor

Existing automated coverage

• QuickActionsIT#infoBox: Inserts the Info Box through the /info quick action and types its message; asserts the info macro source.
• RealtimeWYSIWYGEditorIT#inplaceEditableMacro: Info Box inserted via quick action in the (realtime) editor, content edited, saved; asserts the view-mode content.

Missing automated coverage

• Add a page object for the CKEditor toolbar Insert (the plus icon) dropdown (CKEditorToolBar has none).
• Edit a page in CKEditor, click Insert (the plus icon) > Info Box, type "This is an info macro!" in its content; assert the info box is rendered in the editor.
• Save & View and assert the view page shows a .box.infomessage containing "This is an info macro!".

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/MacroIT.java

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