MDEV-532: Fix some race conditions in test cases.

With MDEV-532, the binlog_checkpoint event is logged asynchronously
from a binlog background thread. This causes some sporadic failures
in some test cases whose output depends on order of events in
binlog.

Fix using an include file that waits until the binlog checkpoint
event has been logged before proceeding with the test case.

mysql-test/suite/multi_source/multisource.test

@@ -169,6 +169,7 @@ select * from db2.t1;
 
 --connection master1
 flush logs;
+--source include/wait_for_binlog_checkpoint.inc
 --save_master_pos
 --connection slave
 --sync_with_master 0, 'master1'