pgsql: pg_plan_advice: Don't suppress NO_GATHER advice for parent rels.

pg_plan_advice: Don't suppress NO_GATHER advice for parent rels.

When a table is partitioned, the previous code did not emit NO_GATHER
advice for the parent table, on the theory that doing this for the
child tables was good enough. However, it isn't. I believe what
happened here is that, during development, I was planning to enforce
NO_GATHER by clearing the consider_parallel flag for the children,
which would have been good enough to ensure that parallelism could
not be used at any higher level of the plan tree. Eventually, I
switched course, but failed to realize that this code needed to be
updated as a result. Do that, and add a regression test.

Backpatch-through: 19
Discussion: http://postgr.es/m/CA+TgmoYYHC4i-Gpzc9-JYHqih6pOK+6LTYFiNR-PioXG1nKkOQ@mail.gmail.com

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/13cd2227c0170f2760c4a79821e2c285171dd57f

Modified Files
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contrib/pg_plan_advice/expected/gather.out | 66 +++++++++++++++++++
contrib/pg_plan_advice/expected/partitionwise.out | 78 ++++++++++++-----------
contrib/pg_plan_advice/pgpa_scan.c | 7 +-
contrib/pg_plan_advice/sql/gather.sql | 26 ++++++++
4 files changed, 135 insertions(+), 42 deletions(-)