Manually delete fixture in `test_mysqli_flush_sync()`.

This test creates a dummy post and subsequently runs a query containing
`DROP PROCEDURE`. This latter query implies a `COMMIT`, which means that the
post is not cleaned up for later tests. Manually deleting the post with
`wp_delete_post()` solves this problem.

Fixes #28155.

git-svn-id: https://develop.svn.wordpress.org/trunk@30320 602fd350-edb4-49c9-b593-d223f7449a82