MTR's internal check of main.log_tables-big failed

The test wasn't restoring log_output properly.
Also added output of query_time in case of wrong result, to
investigate the failure described in MDEV-13408

mysql-test/r/log_tables-big.result

+set @log_output.saved = @@global.log_output;
 set @@global.log_output = 'TABLE';
 set session long_query_time=10;
 select get_lock('bug27638', 1);
@@ -7,25 +8,25 @@ set session long_query_time=1;
 select get_lock('bug27638', 2);
 get_lock('bug27638', 2)
 0
-select if (query_time >= '00:00:01', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:00:01', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
 where sql_text = 'select get_lock(\'bug27638\', 2)';
 qt	sql_text
 OK	select get_lock('bug27638', 2)
 select get_lock('bug27638', 60);
 get_lock('bug27638', 60)
 0
-select if (query_time >= '00:00:59', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:00:59', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
 where sql_text = 'select get_lock(\'bug27638\', 60)';
 qt	sql_text
 OK	select get_lock('bug27638', 60)
 select get_lock('bug27638', 101);
 get_lock('bug27638', 101)
 0
-select if (query_time >= '00:01:40', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:01:40', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
 where sql_text = 'select get_lock(\'bug27638\', 101)';
 qt	sql_text
 OK	select get_lock('bug27638', 101)
 select release_lock('bug27638');
 release_lock('bug27638')
 1
-set @@global.log_output=default;
+set @@global.log_output = @log_output.saved;

mysql-test/t/log_tables-big.test

@@ -7,6 +7,7 @@
 # check that CSV engine was compiled in
 --source include/have_csv.inc
 
+set @log_output.saved = @@global.log_output;
 set @@global.log_output = 'TABLE';
 
 connect (con1,localhost,root,,);
@@ -21,13 +22,13 @@ select get_lock('bug27638', 1);
 connection con2;
 set session long_query_time=1;
 select get_lock('bug27638', 2);
-select if (query_time >= '00:00:01', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:00:01', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
        where sql_text = 'select get_lock(\'bug27638\', 2)';
 select get_lock('bug27638', 60);
-select if (query_time >= '00:00:59', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:00:59', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
        where sql_text = 'select get_lock(\'bug27638\', 60)';
 select get_lock('bug27638', 101);
-select if (query_time >= '00:01:40', 'OK', 'WRONG') as qt, sql_text from mysql.slow_log
+select if (query_time >= '00:01:40', 'OK', concat('WRONG: ',query_time)) as qt, sql_text from mysql.slow_log
        where sql_text = 'select get_lock(\'bug27638\', 101)';
 connection con1;
 select release_lock('bug27638');
@@ -36,4 +37,4 @@ connection default;
 disconnect con1;
 disconnect con2;
 
-set @@global.log_output=default;
+set @@global.log_output = @log_output.saved;