MDEV-25307: The value of the auto-increment variables changes during the test

Part #2, specifically for the 10.5+ branch:

The auto-increment parameters can change sporadically during the
execution of the mtr test "galera_vote_rejoin_ddl", causing it to
fail. This patch creates an environment where unpredictable changes
to these auto-increment settings do not occur during the test.

mysql-test/suite/galera/t/galera_vote_rejoin_ddl.cnf

@@ -2,3 +2,9 @@
 
 [mysqld]
 wsrep-ignore-apply-errors=0
+
+[mysqld.3]
+auto_increment_offset=3
+
+[mysqld.4]
+auto_increment_offset=4