Use `wp_parse_id_list()` when parsing `exclude_tree` in `get_pages()`. Add unit tests to ensure a URL string, array with string as value, and array with array as value for `exclude_tree` can be used to specify multiple IDs.

Props cgaffga, roothorick, hakre, tbrams for patches across the years.
Fixes #9153.


Built from https://develop.svn.wordpress.org/trunk@27767


git-svn-id: http://core.svn.wordpress.org/trunk@27604 1a063a9b-81f0-0310-95a4-ce76da25c4cd