Reinstate term splitting on term update.

Originally introduced in [30241] and reverted in [30336], term splitting is
back and better than ever. Now with *more unit tests* and *improved treatment
of child terms*!

See #30335.

git-svn-id: https://develop.svn.wordpress.org/trunk@30344 602fd350-edb4-49c9-b593-d223f7449a82