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.

Report and patch by David Rowley.

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

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/01474f56981a4b27fe5e304a61b0fce2ac025c69

Modified Files
--------------
src/test/regress/expected/partition_aggregate.out | 114 ++++++++++++----------
src/test/regress/sql/partition_aggregate.sql | 3 +
2 files changed, 63 insertions(+), 54 deletions(-)