MDEV-31570 gap_lock_split.test hangs sporadically

The fix is in replacing the waiting for the whole purge finishing
with the the waiting for only delete-marked records purging finishing.

Reviewed by: Marko Mäkelä

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

 SET @save_frequency=@@GLOBAL.innodb_purge_rseg_truncate_frequency;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=1;
-CREATE TABLE t1(id INT PRIMARY key, val VARCHAR(16000)) ENGINE=InnoDB;
+CREATE TABLE t1(id INT PRIMARY key, val VARCHAR(16000))
+ENGINE=InnoDB STATS_PERSISTENT=0;
 INSERT INTO t1 (id,val) SELECT 2*seq,'x' FROM seq_0_to_1023;
 connect con1,localhost,root,,;
 InnoDB		0 transactions not purged
 START TRANSACTION WITH CONSISTENT SNAPSHOT;
 connection default;
 DELETE FROM t1 WHERE id=1788;
+SET @saved_dbug = @@GLOBAL.debug_dbug;
+SET @@GLOBAL.debug_dbug="d,enable_row_purge_del_mark_exit_sync_point";
 BEGIN;
 SELECT * FROM t1 WHERE id=1788 FOR UPDATE;
 id	val
 connection con1;
 COMMIT;
-InnoDB		0 transactions not purged
+SET DEBUG_SYNC = 'now WAIT_FOR row_purge_del_mark_finished';
+SET @@GLOBAL.debug_dbug = @saved_dbug;
 connection default;
 INSERT INTO t1 (id,val) VALUES (1787, REPEAT('x',2000));
 connection con1;

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

 --source include/have_innodb.inc
 --source include/have_sequence.inc
 --source include/have_debug.inc
+--source include/have_debug_sync.inc
 
 SET @save_frequency=@@GLOBAL.innodb_purge_rseg_truncate_frequency;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=1;
 
-CREATE TABLE t1(id INT PRIMARY key, val VARCHAR(16000)) ENGINE=InnoDB;
+CREATE TABLE t1(id INT PRIMARY key, val VARCHAR(16000))
+  ENGINE=InnoDB STATS_PERSISTENT=0;
 INSERT INTO t1 (id,val) SELECT 2*seq,'x' FROM seq_0_to_1023;
 
 connect(con1,localhost,root,,);
@@ -16,13 +18,18 @@ connection default;
 
 DELETE FROM t1 WHERE id=1788;
 
+SET @saved_dbug = @@GLOBAL.debug_dbug;
+SET @@GLOBAL.debug_dbug="d,enable_row_purge_del_mark_exit_sync_point";
+
 BEGIN;
 # This will return no result, but should acquire a gap lock.
 SELECT * FROM t1 WHERE id=1788 FOR UPDATE;
 
 connection con1;
 COMMIT;
-source include/wait_all_purged.inc;
+SET DEBUG_SYNC = 'now WAIT_FOR row_purge_del_mark_finished';
+SET @@GLOBAL.debug_dbug = @saved_dbug;
+
 connection default;
 
 INSERT INTO t1 (id,val) VALUES (1787, REPEAT('x',2000));