Add automated test for "Add Emoji"

The manual test is here: Add Emoji

Existing automated coverage

• QuickActionsIT#emoji: Inserts an emoji via the /emoji quick action and the ":" autocomplete; asserts the source.
• QuickActionsIT#emojiClickTriggersDropDown: Same path, triggered by clicking the quick action.

Missing automated coverage

• Add a page object for the CKEditor Emoji List toolbar panel.
• Click Emoji List and select an emoji; assert it is inserted.
• Reopen the panel, switch through the category tabs (People, Nature and animals, Food and drinks, Travel and places, Activities, Objects, Symbols, Flags) and select an emoji from one of them; assert it is inserted.
• Reopen the panel, search with "Search emoji..." using a word and select the result; assert it is inserted.
• Save & View and assert all inserted emojis 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 EmojiIT 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).