Fix unstable regression test added by commits 59b71c6f et al.

The query didn't really have a preferred index, leading to platform-
specific choices of which one to use.  Adjust it to make sure tenk1_hundred
is always chosen.

Per buildfarm.

src/test/regress/expected/aggregates.out

@@ -2013,7 +2013,7 @@ CREATE AGGREGATE balk(
     STYPE = int8,
     "PARALLEL" = SAFE,
     INITCOND = '0');
-SELECT balk(1) FROM tenk1;
+SELECT balk(hundred) FROM tenk1;
  balk 
 ------
      
@@ -2047,17 +2047,17 @@ CREATE AGGREGATE balk(
 ALTER TABLE tenk1 set (parallel_workers = 4);
 SET LOCAL parallel_setup_cost=0;
 SET LOCAL max_parallel_workers_per_gather=4;
-EXPLAIN (COSTS OFF) SELECT balk(1) FROM tenk1;
+EXPLAIN (COSTS OFF) SELECT balk(hundred) FROM tenk1;
                                QUERY PLAN                                
---------------------------------------------------------------------------------
+-------------------------------------------------------------------------
  Finalize Aggregate
    ->  Gather
          Workers Planned: 4
          ->  Partial Aggregate
-               ->  Parallel Index Only Scan using tenk1_thous_tenthous on tenk1
+               ->  Parallel Index Only Scan using tenk1_hundred on tenk1
 (5 rows)
 
-SELECT balk(1) FROM tenk1;
+SELECT balk(hundred) FROM tenk1;
  balk 
 ------
      

src/test/regress/sql/aggregates.sql

@@ -868,7 +868,7 @@ CREATE AGGREGATE balk(
     "PARALLEL" = SAFE,
     INITCOND = '0');
 
-SELECT balk(1) FROM tenk1;
+SELECT balk(hundred) FROM tenk1;
 
 ROLLBACK;
 
@@ -902,7 +902,7 @@ ALTER TABLE tenk1 set (parallel_workers = 4);
 SET LOCAL parallel_setup_cost=0;
 SET LOCAL max_parallel_workers_per_gather=4;
 
-EXPLAIN (COSTS OFF) SELECT balk(1) FROM tenk1;
-SELECT balk(1) FROM tenk1;
+EXPLAIN (COSTS OFF) SELECT balk(hundred) FROM tenk1;
+SELECT balk(hundred) FROM tenk1;
 
 ROLLBACK;