mtr test galera.MDEV-20793 has sporadic false positive failures

galera.MDEV-20793 test fails occasionally for result content mismatch of type:

— /home/seppo/work/wsrep/mariadb-server/mysql-test/suite/galera/r/MDEV-20793.result 2021-10-21 17:38:55.815772506 +0300
+++ /home/seppo/work/wsrep/mariadb-server/mysql-test/suite/galera/r/MDEV-20793.reject 2021-12-09 22:42:57.724250085 +0200
@@ -29,6 +29,8 @@
connection node_1_ctrl;
SET GLOBAL wsrep_provider_options = 'signal=apply_monitor_slave_enter_sync';
SET SESSION debug_sync = "now WAIT_FOR reached";
+Warnings:
+Warning 1639 debug sync point wait timed out
SET GLOBAL wsrep_provider_options = 'signal=local_monitor_slave_enter_sync';
SET GLOBAL wsrep_provider_options = 'dbug=';
SET SESSION debug_sync = "now SIGNAL continue";

mysqltest: Result length mismatch