Correct instability in percona_changed_page_bmp_shutdown_thread

mysql-test/mtr innodb.percona_changed_page_bmp_shutdown_thread --valgrind --repeat=10

[ 50%] innodb.percona_changed_page_bmp_shutdown_thread w5 [ fail ]  Found warnings/errors in server log file!
        Test ended at 2020-01-21 10:41:42
line
2020-01-21T10:41:21.847783Z 0 [Warning] InnoDB: Last tracked LSN in './ib_modified_log_2_1355560.xdb' is 1355560, but the last checkpoint LSN is 1355569.  This might be due to a server crash or a very fast shutdown.
^ Found warnings in /tmp/ps/bld/mysql-test/var/5/log/mysqld.1.err
ok