MDEV-11491 binlog_encryption.rpl_checksum fails sporadically in buildbot

The race condition happened if mark_xid_done was considerably delayed,
and an extra Binlog_checkpoint event was written into the binary log
which was later indicated in an error message. Fixed by ensuring
that the event is written before the binary log is rotated to the one
which is used in the output.

mysql-test/extra/rpl_tests/rpl_checksum.inc

@@ -87,6 +87,7 @@ create table t1 (a int);
 # testing that binlog rotation preserves opt_binlog_checksum value
 flush logs;
 flush logs;
+-- source include/wait_for_binlog_checkpoint.inc
 flush logs;
 
 sync_slave_with_master;