Add supression for warning.

mysql-test/suite/galera/r/galera_ssl_upgrade.result

+call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown");
 SELECT VARIABLE_VALUE = 'Synced' FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_local_state_comment';
 VARIABLE_VALUE = 'Synced'
 1

mysql-test/suite/galera/t/galera_ssl_upgrade.test

@@ -8,6 +8,8 @@
 --source include/have_innodb.inc
 --source include/have_ssl_communication.inc
 
+call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown");
+
 SELECT VARIABLE_VALUE = 'Synced' FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_local_state_comment';
 SELECT VARIABLE_VALUE = 2 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_cluster_size';