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

Fixes #9470.


Built from https://develop.svn.wordpress.org/trunk@25244


git-svn-id: http://core.svn.wordpress.org/trunk@25212 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Set `hierarchical` to `false` when `parent` is passed to `get_pages()` and `child_of` is not.

Fixes #25230.


Built from https://develop.svn.wordpress.org/trunk@25245


git-svn-id: http://core.svn.wordpress.org/trunk@25213 1a063a9b-81f0-0310-95a4-ce76da25c4cd