Fix sporadic test failures in rpl_mariadb_slave_capability.test.

The problem is the async binlog checkpointing; this could on rare
occasions occur too late, causing SHOW BINLOG EVENTS to show the
wrong events and cause .result file difference.

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

@@ -58,6 +58,7 @@ set @@global.debug_dbug= @old_slave_dbug;
 
 connection master;
 set @@global.binlog_checksum = CRC32;
+--source include/wait_for_binlog_checkpoint.inc
 TRUNCATE t1;
 let $binlog_file= query_get_value(SHOW MASTER STATUS, File, 1);
 let $binlog_start= query_get_value(SHOW MASTER STATUS, Position, 1);