Docs: Add missing parameter description in `wp_get_post_parent_id()` function DocBlock.

Rename `$post_ID` to `$post` for consistency with other functions.

Add unit tests for `wp_get_post_parent_id()`.

Props keesiemeijer.
Fixes #42797.
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git-svn-id: https://core.svn.wordpress.org/trunk@42226 1a063a9b-81f0-0310-95a4-ce76da25c4cd