pgsql: Force parallelism in partition_aggregate

Force parallelism in partition_aggregate

Commit db0d67db2 tweaked sort costing, which however resulted in a
couple plan changes in our regression tests. Most of the new plans were
fine, but partition_aggregate were meant to test parallel plans and the
new plans were serial.

Fix that by lowering parallel_setup_cost to 0, which is enough to switch
to the parallel plan again.

Commit 1349d2790 already made the plans parallel again, but do this
anyway to keep the tests in sync with 15, to make backpatching simpler.

Report and patch by David Rowley.

Author: David Rowley
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAApHDvpVFgWzXdtUQkjyOPhNrNvumRi_=ftgS79KeAZ92tnHKQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2fe6b2a806f20f2dbf02c6e79783dbc2777a1840

Modified Files
--------------
src/test/regress/expected/partition_aggregate.out | 2 ++
src/test/regress/sql/partition_aggregate.sql | 3 +++
2 files changed, 5 insertions(+)