Add automated test for "Insert Special Character"

The manual test is here: Insert Special Character

Existing automated coverage

None found.
Searched for specialchar / Special Character in all Java and JavaScript tests and page objects (ckeditor, realtime, edit modules) and the CKEditor toolbar page object.

Missing automated coverage

• Add a page object for the Insert (the plus icon) menu and the CKEditor Special Character dialog.
• Edit a page in CKEditor, open Insert (the plus icon) > Special Character and click a few characters (e.g. ©, €, ½).
• Assert the characters are inserted in the edited content / source.
• Save & View and assert the characters are displayed 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/QuickActionsIT.java (or a new SpecialCharIT test class in xwiki-platform-core/xwiki-platform-ckeditor/xwiki-platform-ckeditor-test/xwiki-platform-ckeditor-test-docker)

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