Add automated test for "Copy and Paste text"

The manual test is here: Copy and Paste text

Existing automated coverage

• ImageIT#moveImageWithinEditedContentWithCopyPaste: Cut (Ctrl+X) and paste (Ctrl+V) of a text+image selection inside the editor; asserts the resulting source.
• ImageIT#uploadImagesFromPastedHTML: Pasting HTML copied from another page into the editor.

Missing automated coverage

• Edit a page in CKEditor and enter (or paste) a multi-paragraph block of text.
• Select a few words, press Ctrl+C, move the caret to another paragraph, press Ctrl+V.
• Assert the source contains the copied text at both places (the original kept) and nothing else changed.
• Save & View and assert the pasted text is displayed.

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/FilterIT.java (or a new ClipboardIT 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).