pgsql: Improve coverage of nodeAppend runtime partition prune

Improve coverage of nodeAppend runtime partition prune

coverage report indicated that mark_invalid_subplans_as_finished() and
nearby code was not getting exercised by any tests. Add a new one which
has execution-time Params rather than only external Params to fix this.

In passing, David noticed that ab_q6 tests were not actually required to
have a generic plan. The tests were testing exec Params not external
Params, so there was no need for the PREPARE. Remove the PREPARE,
making these plain queries. (The new queries are called from
explain_parallel_append, which may be unnecessary since they don't
actually have a Parallel Append node, just an Append. But it doesn't
seem to hurt anything, either.)

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f--hopb6JBSDY4wiXTS3ZcDp-wparXjTQ1nzNdBa04Fog@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/95cdc77b35ece28137b8dec8c6f9b5474b215624

Modified Files
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src/test/regress/expected/partition_prune.out | 92 ++++++++++++---------------
src/test/regress/sql/partition_prune.sql | 37 +++++------
2 files changed, 55 insertions(+), 74 deletions(-)