pgsql: Add INSERT ON CONFLICT test on partitioned tables withtransitio

Add INSERT ON CONFLICT test on partitioned tables with transition table

This case was uncovered by existing tests, so breakage went undetected.
Make sure it remains stable.

Extracted from a larger patch by
Author: David Rowley
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/CAKJS1f-aGCJ5H7_hiSs5PhWs6Obmj+vGARjGymqH1=o5PcrNnQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9079fe60b2142da6e89403b9a9504e75124c085d

Modified Files
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src/test/regress/expected/insert_conflict.out | 22 ++++++++++++++++++++++
src/test/regress/sql/insert_conflict.sql      | 26 ++++++++++++++++++++++++++
2 files changed, 48 insertions(+)