[PP-1] Add BookTestTrait::addNodeToBook() method and convert book tests to use it

This extension is being deprecated, see 🌱 [Meta] Tasks to deprecate Book module Active . It will be removed from core and moved to a contrib project, 📌 [11.x] [Meta] Tasks to remove Book Active .

This is now Postponed. The status is set according to two policies. The Remove a core extension and move it to a contributed project → and the Extensions approved for removal → policies.

This issue may be re-opened if it can be considered critical → , If unsure, re-open the issue and ask in a comment.

Moving to contrib