MDEV-11082 mysql_client_test: test_ps_query_cache fails with group-concat-max-len=1M

test_bug14169 was setting session group_concat_max_len=1024 and
did not clean it up. Because of that test_ps_query_cache, when run
with group-concat-max-len != 1024, had different values in connections,
and was inserting into query cache when a hit was expected.

Fixed by adding a clean-up for the value in test_bug14169

tests/mysql_client_test.c

@@ -15375,6 +15375,9 @@ static void test_bug14169()
 
   rc= mysql_query(mysql, "drop table t1");
   myquery(rc);
+
+  rc= mysql_query(mysql, "set session group_concat_max_len=@@global.group_concat_max_len");
+  myquery(rc);
 }
 
 /*