MDEV-5999 MySQL Bug#12766319 - 61865: RENAME USER DOES NOT WORK CORRECTLY - REQUIRES FLUSH PRIVILEGES

use update_hostname() to update the hostname.
test case comes from

  commit 0abdeed1d6d
  Author: gopal.shankar@oracle.com <>
  Date:   Thu Mar 29 00:20:54 2012 +0530

    Bug#12766319 - 61865: RENAME USER DOES NOT WORK CORRECTLY -
                          REQUIRES FLUSH PRIVILEGES