Fix merge-big.test to properly restore debug settings.

Otherwise following tests go crazy in the log, writing gigabytes of data
and causing havoc.

mysql-test/r/merge-big.result

@@ -7,6 +7,7 @@ drop table if exists t1,t2,t3,t4,t5,t6;
 CREATE TABLE t1 (c1 INT) ENGINE= MyISAM;
 LOCK TABLE t1 WRITE;
 # connection con1
+SET @orig_debug=@@debug;
 SET GLOBAL debug="+d,sleep_open_and_lock_after_open";
 INSERT INTO t1 VALUES (1);
 # connection default
@@ -21,6 +22,6 @@ SELECT * FROM t1;
 c1
 UNLOCK TABLES;
 # connection con1
-SET GLOBAL debug="-d,sleep_open_and_lock_after_open";
+SET GLOBAL debug=@orig_debug;
 # connection default
 DROP TABLE t1;

mysql-test/t/merge-big.test

@@ -43,6 +43,7 @@ LOCK TABLE t1 WRITE;
     --echo # connection con1
     connect (con1,localhost,root,,);
     let $con1_id= `SELECT CONNECTION_ID()`;
+    SET @orig_debug=@@debug;
     SET GLOBAL debug="+d,sleep_open_and_lock_after_open";
     send INSERT INTO t1 VALUES (1);
 --echo # connection default
@@ -74,7 +75,7 @@ UNLOCK TABLES;
     --echo # connection con1
     connection con1;
     reap;
-    SET GLOBAL debug="-d,sleep_open_and_lock_after_open";
+    SET GLOBAL debug=@orig_debug;
     disconnect con1;
 --echo # connection default
 connection default;