`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.

Built from https://develop.svn.wordpress.org/trunk@30297


git-svn-id: http://core.svn.wordpress.org/trunk@30296 1a063a9b-81f0-0310-95a4-ce76da25c4cd