Minor: Fix invalid query in test

[alamb]

Which issue does this PR close?

• Related to Expand wildcard to actual expressions in prepare_select_exprs #15090

Rationale for this change

In #15090 @jayzhan211 and @goldmedal found that a test I had added in a previous PR is actually invalid

What changes are included in this PR?

Fix query to be valid

Are these changes tested?

Are there any user-facing changes?

[alamb]

This query is not valid as b does not appear in the subquery select lists

Postgres agrees

postgres=# create table t(a0 int, a int, b int, c int);
CREATE TABLE
postgres=# insert into t values (1, 2, 3, 4);
INSERT 0 1
postgres=# insert into t values (5, 6, 7, 8);
INSERT 0 1
postgres=# select * from (select c, a, NULL::int as a0 from t order by a, c) t1
union all
select * from (select c, NULL::int as a, a0 from t order by a0, c) t2
order by c, a, a0, b
limit 2;
ERROR:  column "b" does not exist
LINE 4: order by c, a, a0, b
                           ^

The b needs to be in the subquery select list:

postgres=# select * from (select c, a, NULL::int as a0, b from t order by a, c) t1
union all
select * from (select c, NULL::int as a, a0, b from t order by a0, c) t2
order by c, a, a0, b
limit 2;
 c | a | a0 | b
---+---+----+---
 4 | 2 |    | 3
 4 |   |  1 | 3

[jayzhan211]

Thanks @alamb