Add automated test for "Insert Display Macro"

The manual test is here: Insert Display Macro

Existing automated coverage

• DisplayMacroTest#executeShowsVelocityMacrosAreIsolated (and the other execute tests): Server-side execution of the display macro (unit level).

Missing automated coverage

• Edit a page in CKEditor, Insert (the plus icon) > Other Macros, search "Display" and select it.
• Fill the reference parameter with an existing page and Submit; assert the displayed page content is rendered in the editor.
• Save & View and assert the displayed page content is rendered in view mode.

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).