Add cleanup of mysql.gtid_slave_pos table.

(Found during MDEV-12179 development, but independent of it, so put
into a separate commit).

mysql-test/suite/rpl/r/rpl_mdev10863.result

@@ -46,5 +46,6 @@ SET GLOBAL slave_parallel_threads=@old_parallel_threads;
 SET GLOBAL max_relay_log_size= @old_max_relay;
 include/start_slave.inc
 connection server_1;
+ALTER TABLE mysql.gtid_slave_pos ENGINE=MyISAM;
 DROP TABLE t1;
 include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_mdev10863.test

@@ -99,6 +99,7 @@ SET GLOBAL max_relay_log_size= @old_max_relay;
 --source include/start_slave.inc
 
 --connection server_1
+ALTER TABLE mysql.gtid_slave_pos ENGINE=MyISAM;
 DROP TABLE t1;
 
 --source include/rpl_end.inc