MDEV-16951: binlog_encryption.rpl_checksum failed in buildbot with wrong result

Wait for the binlog checkpoint event to fix non-determinism in the testcase.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

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

@@ -89,6 +89,9 @@ flush logs;
 flush logs;
 -- source include/wait_for_binlog_checkpoint.inc
 flush logs;
+# The binlog position here is output in the error message from
+# wait_for_slave_io_error below, so make sure it's deterministic.
+-- source include/wait_for_binlog_checkpoint.inc
 
 sync_slave_with_master;
 #connection slave;