MDEV-33285 make the test deterministic

need to clear old signals
before waiting for new signals with the same name

mysql-test/main/kill_debug.result

@@ -252,6 +252,7 @@ KILL QUERY id;
 SET debug_sync='now SIGNAL go';
 connection default;
 ERROR 70100: Query execution was interrupted
+SET debug_sync='RESET';
 SET debug_sync='mysql_checksum_table_before_calculate_checksum SIGNAL parked WAIT_FOR go';
 CHECKSUM TABLE t1;
 connection con1;

mysql-test/main/kill_debug.test

@@ -338,6 +338,7 @@ connection default;
 error ER_QUERY_INTERRUPTED;
 reap;
 
+SET debug_sync='RESET';
 SET debug_sync='mysql_checksum_table_before_calculate_checksum SIGNAL parked WAIT_FOR go';
 send CHECKSUM TABLE t1;
 connection con1;