When passing $full to get_posts_by_author_sql(), make sure a 'post_type' clause is included in results.
This change makes the 'post_type' clause in wp_list_authors() redundant, so
we remove it. Third-party plugins using get_posts_by_author_sql() may have
similarly redundant clauses, but this won't change the results returned by the
SQL queries.
Also adds unit tests for get_posts_by_author_sql().