Modify an incorrect regression test case in the group by key value elimination function

Dear hackers: 


Two months ago, we enhanced the group by key value elimination function. This is a very useful function. When I was learning, I found a regression test case that was not suitable, as follows:


-- When there are multiple supporting unique indexes and the GROUP BY contains
-- columns to cover all of those, ensure we pick the index with the least
-- number of columns so that we can remove more columns from the GROUP BY.
explain (costs off) select a,b,c from t3 group by a,b,c;
      QUERY PLAN      
----------------------
 HashAggregate
   Group Key: c
   ->  Seq Scan on t3
(3 rows)


-- As above but try ordering the columns differently to ensure we get the
-- same result.
explain (costs off) select a,b,c from t3 group by c,a,b;
      QUERY PLAN      
----------------------
 HashAggregate
   Group Key: c
   ->  Seq Scan on t3
(3 rows)


Because the table structure of t3 is defined as follows(Its PK is deferrable):


postgres=# \d+ t3
                                          Table "pg_temp_1.t3"
 Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
 a      | integer |           | not null |         | plain   |             |              |
 b      | integer |           | not null |         | plain   |             |              |
 c      | integer |           | not null |         | plain   |             |              |
Indexes:
    "t3_pkey" PRIMARY KEY, btree (a, b) DEFERRABLE
    "t3_c_uidx" UNIQUE, btree (c)
Not-null constraints:
    "t3_a_not_null" NOT NULL "a"
    "t3_b_not_null" NOT NULL "b"
    "t3_c_not_null" NOT NULL "c"
Access method: heap


postgres=#


I think this test case does not fully reflect the original meaning. So I made a small change to this and look forward to your feedback. Thanks!