Tests: Further fix some issues with indeterminate sort ordering in `Tests_List_Pages`:

* Make sure `test_wp_list_pages_number()` sorts by ID, as there are several pages with the same title.
* Limit `test_wp_list_pages_sort_column()` to one level, as the child page fixtures don't have an author and cannot be reliably sorted by `post_author`.

Follow-up to [48157].

Props afercia.
See #50466.
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git-svn-id: https://core.svn.wordpress.org/trunk@48049 1a063a9b-81f0-0310-95a4-ce76da25c4cd