Modify MTR test to prevent proxy_protocol_admin_port test failure on 8.0.21

200907 14:14:51 [ 90%] main.proxy_protocol_admin_port w6 [ fail ] Found warnings/errors in error log file!
Test ended at 2020-09-07 14:14:51
line
2020-09-07T14:14:50.701673Z 0 [Warning] [MY-013595] [Server] Failed to initialize TLS for channel: mysql_admin. See below for the description of exact issue.
2020-09-07T14:14:50.702034Z 0 [Warning] [MY-010069] [Server] Failed to set up SSL because of the following SSL library error: SSL context is not usable without certificate and private key
SET @@GLOBAL.super_read_only = @original_super_read_only;
^ Found warnings in /tmp/results/PS/mysql-test/var/6/log/mysqld.1.err
ok