pgsql: Fix unstable aggregate regression test

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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d6a6957d53e3de9982b076b6ddd703be3d0ae30a

Modified Files
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src/test/regress/expected/aggregates.out | 2 +-
src/test/regress/sql/aggregates.sql      | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)