MDEV-12754 innodb.truncate_debug fails in buildbot with embedded due to semaphore wait

fix sporadic innodb.truncate_purge_debug failures

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

@@ -75,12 +75,12 @@ connect  con2,localhost,root,,;
 set global innodb_adaptive_hash_index=off;
 connection default;
 SET DEBUG_SYNC= 'now SIGNAL finish_scan';
-SET DEBUG_SYNC= 'RESET';
 connection con1;
 disconnect con1;
 connection con2;
 disconnect con2;
 connection default;
+SET DEBUG_SYNC= 'RESET';
 SET session lock_wait_timeout=default;
 set global innodb_adaptive_hash_index=on;
 drop table t1,t2,t3;

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

@@ -22,8 +22,8 @@ connection default;
 InnoDB		0 transactions not purged
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;
 SET DEBUG_SYNC = 'now SIGNAL finish_scan';
-SET DEBUG_SYNC = 'RESET';
 connection con1;
 disconnect con1;
 connection default;
+SET DEBUG_SYNC = 'RESET';
 drop table t1;

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

@@ -103,7 +103,6 @@ send set global innodb_adaptive_hash_index=off;
 
 connection default;
 SET DEBUG_SYNC= 'now SIGNAL finish_scan';
-SET DEBUG_SYNC= 'RESET';
 
 connection con1;
 reap;
@@ -114,6 +113,7 @@ reap;
 disconnect con2;
 
 connection default;
+SET DEBUG_SYNC= 'RESET';
 
 SET session lock_wait_timeout=default;
 set global innodb_adaptive_hash_index=on;

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

@@ -55,12 +55,12 @@ echo $remaining transactions not purged;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;
 
 SET DEBUG_SYNC = 'now SIGNAL finish_scan';
-SET DEBUG_SYNC = 'RESET';
 
 connection con1;
 reap;
 disconnect con1;
 
 connection default;
+SET DEBUG_SYNC = 'RESET';
 drop table t1;
 --source include/wait_until_count_sessions.inc