Allow `get_pages()` to accept an `array` for the arg `parent`. Adds unit tests, some will fail until the next commit.

Fixes #9470.



git-svn-id: https://develop.svn.wordpress.org/trunk@25244 602fd350-edb4-49c9-b593-d223f7449a82
Set `hierarchical` to `false` when `parent` is passed to `get_pages()` and `child_of` is not.

Fixes #25230.



git-svn-id: https://develop.svn.wordpress.org/trunk@25245 602fd350-edb4-49c9-b593-d223f7449a82