Fix unstable aggregate regression test

Buildfarm member avocet has shown a plan change by switching the
finalize aggregate stage to use a GroupAggregate rather than a
HashAggregate.  This is consistent with autovacuum having triggered on
the table, per analysis by Alexander Lakhin.

Fix this by disabling autovacuum on the table.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/d4493a28-589a-5328-fed5-250f2d7d3e2a@gmail.com
Backpatch-through: 16, where this test was added.

src/test/regress/expected/aggregates.out

@@ -1930,7 +1930,7 @@ select string_agg(v, decode('ee', 'hex')) from bytea_test_table;
 
 drop table bytea_test_table;
 -- Test parallel string_agg and array_agg
-create table pagg_test (x int, y int);
+create table pagg_test (x int, y int) with (autovacuum_enabled = off);
 insert into pagg_test
 select (case x % 4 when 1 then null else x end), x % 10
 from generate_series(1,5000) x;

src/test/regress/sql/aggregates.sql

@@ -749,7 +749,7 @@ select string_agg(v, decode('ee', 'hex')) from bytea_test_table;
 drop table bytea_test_table;
 
 -- Test parallel string_agg and array_agg
-create table pagg_test (x int, y int);
+create table pagg_test (x int, y int) with (autovacuum_enabled = off);
 insert into pagg_test
 select (case x % 4 when 1 then null else x end), x % 10
 from generate_series(1,5000) x;