Taxonomy: Ensure the `child_of` argument of `get_terms()` works as expected...

Taxonomy: Ensure the `child_of` argument of `get_terms()` works as expected with `'fields' => 'id=>name'` or `'id=>slug'`.

Props Howdy_McGee, deepaklalwani, planvova.
Fixes #46768.
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git-svn-id: https://core.svn.wordpress.org/trunk@48425 1a063a9b-81f0-0310-95a4-ce76da25c4cd