Follow-up patch:

 - innobase_flush_logs() always flush the log to disk independently of
   @@innodb_flush_log_at_trx_commit setting (in Mariadb, innobase_flush_logs()
   is not used in connection with commit).

 - Add mtr testcase.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>