Tests: Add a post author to "Parent 1" page fixture in `tests/post/listPages.php`.

This avoids indeterminate ordering in `test_wp_list_pages_sort_column()`.

Fixes #50466.
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git-svn-id: https://core.svn.wordpress.org/trunk@47926 1a063a9b-81f0-0310-95a4-ce76da25c4cd