MDEV-12288 follow-up: Adjust a test for the purge of insert_undo log

The inserted SYS_INDEXES record for the ADD INDEX operation will also
be counted as an un-purgeable transaction.

mysql-test/suite/gcol/r/innodb_virtual_debug_purge.result

@@ -115,7 +115,7 @@ INSERT INTO t1(a, b) VALUES (8, 8);
 # enable purge
 COMMIT;
 # wait for purge to process the deleted/updated records.
-InnoDB		1 transactions not purged
+InnoDB		2 transactions not purged
 SET DEBUG_SYNC= 'now SIGNAL purged';
 disconnect con1;
 connection default;
@@ -149,7 +149,7 @@ disconnect prevent_purge;
 connection default;
 select variable_value>1 from information_schema.global_status where variable_name='innodb_purge_trx_id_age';
 variable_value>1
-1
+0
 disconnect con1;
 start transaction with consistent snapshot;
 commit;