Bug#32108 - subselect.test produces warnings files

Comment sign of -- at line begin in test files lead to warnings
from mysqltest.
  
Changed -- to #.

mysql-test/t/subselect.test

@@ -2970,7 +2970,7 @@ DROP TABLE t1,t2;
 CREATE TABLE t1 (a INT, b INT);
 INSERT INTO t1 VALUES (1, 2), (1,3), (1,4), (2,1), (2,2);
 
--- returns no rows, when it should
+# returns no rows, when it should
 SELECT a1.a, COUNT(*) FROM t1 a1 WHERE a1.a = 1
 AND EXISTS( SELECT a2.a FROM t1 a2 WHERE a2.a = a1.a)
 GROUP BY a1.a;