MDEV-32050 fixup: innodb.instant_alter_crash

This test occasionally fails with a failure to purge history.
Let us try to purge everything before starting the interesting part,
to make that occasional failure go away.

mysql-test/suite/innodb/r/instant_alter_crash.result

@@ -27,6 +27,7 @@ SELECT * FROM t2;
 id	c2	c3
 2	1	De finibus bonorum
 3	4	accusantium doloremque laudantium
+InnoDB		0 transactions not purged
 BEGIN;
 DELETE FROM t1;
 ROLLBACK;

mysql-test/suite/innodb/t/instant_alter_crash.test

@@ -38,6 +38,7 @@ disconnect ddl;
 
 SELECT * FROM t1;
 SELECT * FROM t2;
+--source include/wait_all_purged.inc
 BEGIN;
 DELETE FROM t1;
 ROLLBACK;