MDEV-13549: Galera test failures

Fix test case galera.pxc-421 to reset auto_increment_offset
correctly.

mysql-test/suite/galera/r/pxc-421.result

@@ -26,7 +26,6 @@ SET GLOBAL wsrep_slave_threads = 1;
 SELECT COUNT(*) FROM t1;
 COUNT(*)
 16
-SET GLOBAL auto_increment_offset = 2;
 SELECT COUNT(*) FROM t1;
 COUNT(*)
 15
@@ -35,4 +34,3 @@ Variable_name	Value
 wsrep_slave_threads	12
 SET GLOBAL wsrep_slave_threads = 1;
 DROP TABLE t1;
-SET GLOBAL auto_increment_offset = 1;

mysql-test/suite/galera/t/pxc-421.test

@@ -6,6 +6,11 @@
 --source include/galera_cluster.inc
 --source include/have_innodb.inc
 
+# Save original auto_increment_offset values.
+--let $node_1=node_1
+--let $node_2=node_2
+--source include/auto_increment_offset_save.inc
+
 --connection node_1
 --let $wsrep_slave_1 = `SELECT @@wsrep_slave_threads`
 set GLOBAL wsrep_slave_threads=26;
@@ -54,12 +59,11 @@ show global variables like 'wsrep_slave_threads';
 --eval SET GLOBAL wsrep_slave_threads = $wsrep_slave_2
 SELECT COUNT(*) FROM t1;
 
-SET GLOBAL auto_increment_offset = 2;
-
 --connection node_1
 SELECT COUNT(*) FROM t1;
 show global variables like 'wsrep_slave_threads';
 --eval SET GLOBAL wsrep_slave_threads = $wsrep_slave_1
 DROP TABLE t1;
 
-SET GLOBAL auto_increment_offset = 1;
+--source include/auto_increment_offset_restore.inc
+