MDEV-13256 innodb.truncate_debug fails in buildbot

Ensure that no adaptive hash index exists for any system tables,
so that the blocked TRUNCATE TABLE t1 will not block the concurrent
TRUNCATE TABLE t2.

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

 #
 # Bug #23070734	CONCURRENT TRUNCATE TABLES CAUSE STALLS
 #
+SET @ahi= @@global.innodb_adaptive_hash_index;
+SET GLOBAL innodb_adaptive_hash_index=OFF;
+SET GLOBAL innodb_adaptive_hash_index=ON;
 Test_1 :- Check if DDL operations are possible on
 table being truncated. Also check if
 DDL operations on other tables succeed.
@@ -81,6 +84,5 @@ connection con2;
 disconnect con2;
 connection default;
 SET DEBUG_SYNC= 'RESET';
-SET session lock_wait_timeout=default;
-set global innodb_adaptive_hash_index=on;
+SET GLOBAL innodb_adaptive_hash_index=@ahi;
 drop table t1,t2,t3;

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

@@ -8,6 +8,12 @@
 --echo # Bug #23070734	CONCURRENT TRUNCATE TABLES CAUSE STALLS
 --echo #
 
+SET @ahi= @@global.innodb_adaptive_hash_index;
+# Ensure that there is no adaptive hash index on any system tables,
+# or any other tables than the ones that we are creating below.
+SET GLOBAL innodb_adaptive_hash_index=OFF;
+SET GLOBAL innodb_adaptive_hash_index=ON;
+
 --echo Test_1 :- Check if DDL operations are possible on
 --echo		 table being truncated. Also check if
 --echo		 DDL operations on other tables succeed.
@@ -115,8 +121,7 @@ disconnect con2;
 connection default;
 SET DEBUG_SYNC= 'RESET';
 
-SET session lock_wait_timeout=default;
-set global innodb_adaptive_hash_index=on;
+SET GLOBAL innodb_adaptive_hash_index=@ahi;
 
 drop table t1,t2,t3;
 --source include/wait_until_count_sessions.inc