`wpdb::flush()` was not flushing results correctly when using mysqli.

This change also allows stored procedures or queries made with `mysqli_multi_query()` to be flushed.

Includes unit tests.

Fixes #28155.

Props soulseekah.


git-svn-id: https://develop.svn.wordpress.org/trunk@30297 602fd350-edb4-49c9-b593-d223f7449a82