In `get_terms()`, leverage `get_term_children()` over `_get_term_children()` when making sure to show empty terms that have children in a hierarchical taxonomy while avoiding duplicates.

Adds unit test for `child_of` param. Adjusts unit tests for `get_terms()`.

See [27108] and [27125].
Props SergeyBiryukov.
Fixes #27123.



git-svn-id: https://develop.svn.wordpress.org/trunk@27458 602fd350-edb4-49c9-b593-d223f7449a82