MDEV-21546 main.backup_stages occasionally reports lock wait timeout

With MDEV-16678, InnoDB background tasks (most notably, the purge of
committed transaction history) can acquire metadata locks.
Because of this, the lock_wait_timeout=0 is too strict and must
be relaxed.

The test used to fail easily if an extra sleep was added to
the end of dict_table_close(), before the MDL release. Now,
with lock_wait_timeout=1, the test passes even with an extra
0.1-second sleep added to dict_table_close().

Thanks to Monty for providing this fix.

mysql-test/main/backup_stages.result

@@ -63,7 +63,7 @@ INSERT INTO t_permanent_myisam SET col1 = 1;
 connect backup,localhost,root,,;
 connect con11,localhost,root,,;
 SET AUTOCOMMIT = 0;
-set session lock_wait_timeout=0;
+set session lock_wait_timeout=1;
 connect con12,localhost,root,,;
 SET AUTOCOMMIT = 1;
 # Between (connection default) BACKUP STAGE START and FLUSH
@@ -208,7 +208,7 @@ ERROR HY000: Lock wait timeout exceeded; try restarting transaction
 DROP TABLE t_con1_myisam;
 ERROR HY000: Lock wait timeout exceeded; try restarting transaction
 connection con12;
-SET STATEMENT lock_wait_timeout=0 FOR UPDATE t_permanent_innodb SET col1 = 9;
+SET STATEMENT lock_wait_timeout=1 FOR UPDATE t_permanent_innodb SET col1 = 9;
 ERROR HY000: Lock wait timeout exceeded; try restarting transaction
 connection backup;
 BACKUP STAGE END;

mysql-test/main/backup_stages.test

@@ -119,7 +119,7 @@ INSERT INTO t_permanent_myisam SET col1 = 1;
 --connect(backup,localhost,root,,)
 --connect(con11,localhost,root,,)
 SET AUTOCOMMIT = 0;
-set session lock_wait_timeout=0;
+set session lock_wait_timeout=1;
 --connect(con12,localhost,root,,)
 SET AUTOCOMMIT = 1;
 
@@ -244,7 +244,7 @@ DROP TABLE t_con1_innodb;
 DROP TABLE t_con1_myisam;
 --connection con12
 --error ER_LOCK_WAIT_TIMEOUT
-SET STATEMENT lock_wait_timeout=0 FOR UPDATE t_permanent_innodb SET col1 = 9;
+SET STATEMENT lock_wait_timeout=1 FOR UPDATE t_permanent_innodb SET col1 = 9;
 
 --connection backup
 BACKUP STAGE END;