getPages.php

<?php
/*
 * If hierarchical is false, and child_of is not empty, pages will (apparently) be returned
 * hierarchically anyway in order of creation: 3, 4, 5.
 */
$this->assertEqualSets( array( $page_3, $page_4, $page_5 ), wp_list_pluck( $pages, 'ID' ) );

// How it should work.
$found_pages = wp_list_filter( $pages, array( 'post_parent' => $page_1 ) );
$this->assertEqualSets( array( $page_3, $page_5 ), wp_list_pluck( $found_pages, 'ID' ) );