Add unit tests for WP_Comment_Query 'orderby' param.

For better testability, the SQL string generated in `WP_Comment_Query::get_posts()`
is now stored as a 'request' property on the object.

See #29902.

git-svn-id: https://develop.svn.wordpress.org/trunk@30003 602fd350-edb4-49c9-b593-d223f7449a82