MDEV-14209 innodb_gis.rtree_debug produces huge server error logs

Do not SET DEBUG_DBUG=-d,... in tests. To disable debug instrumentation,
save and restore the original value of the variable DEBUG_DBUG.
Assigning -d,... will enable the output of a lot of unrelated DBUG
messages to the server error log.

mysql-test/suite/innodb_gis/r/rtree_debug.result

@@ -38,14 +38,11 @@ t1	CREATE TABLE `t1` (
   `c2` geometry NOT NULL,
   SPATIAL KEY `idx` (`c2`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
-SET DEBUG='+d,row_merge_ins_spatial_fail';
-Warnings:
-Warning	1287	'@@debug' is deprecated and will be removed in a future release. Please use '@@debug_dbug' instead
+SET @save_dbug = @@SESSION.debug_dbug;
+SET debug_dbug='+d,row_merge_ins_spatial_fail';
 create spatial index idx2 on t1(c2);
 ERROR HY000: Got error 1000 "Unknown error 1000" from storage engine InnoDB
-SET DEBUG='-d,row_merge_ins_spatial_fail';
-Warnings:
-Warning	1287	'@@debug' is deprecated and will be removed in a future release. Please use '@@debug_dbug' instead
+SET debug_dbug = @save_dbug;
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (

mysql-test/suite/innodb_gis/t/rtree_debug.test

@@ -49,10 +49,11 @@ create spatial index idx on t1(c2);
 
 show create table t1;
 
-SET DEBUG='+d,row_merge_ins_spatial_fail';
+SET @save_dbug = @@SESSION.debug_dbug;
+SET debug_dbug='+d,row_merge_ins_spatial_fail';
 --error ER_GET_ERRNO
 create spatial index idx2 on t1(c2);
-SET DEBUG='-d,row_merge_ins_spatial_fail';
+SET debug_dbug = @save_dbug;
 show create table t1;
 
 # Check table.

mysql-test/suite/innodb_gis/t/rtree_search.test

@@ -38,9 +38,10 @@ select count(*) from t1 where MBRWithin(t1.c2, @g1);
 set @g1 = ST_GeomFromText('Polygon((100 100,100 800,800 800,800 100,100 100))');
 select count(*) from t1 where MBRWithin(t1.c2, @g1);
 
-#SET DEBUG='+d, rtr_pessimistic_position';
+#SET @save_dbug= @@session.debug_dbug;
+#SET debug_dbug = '+d,rtr_pessimistic_position';
 #select count(*) from t1 where MBRWithin(t1.c2, @g1);
-#SET DEBUG='-d, rtr_pessimistic_position';
+#SET debug_dbug = @save_dbug;
 
 # Equality search
 set @g1 =  ST_GeomFromText('Point(1 1)');