Add regression test for ATTACH PARTITION

Add regression test for ATTACH PARTITION

This test case uses a SQL function as partitioning operator, whose
evaluation results in the table's relcache being rebuilt partway
through the execution of an ATTACH PARTITION command.

It is extracted from 39808e8, which fixed a bug where this test case
failed on master and REL_11_STABLE, but passed on REL_10_STABLE.  The
partitioning code has changed a lot during v11 development, so this
makes sure that any patch applied to REL_10_STABLE fixing a
partition-related bug does not break it again.

Author: Amit Langote
Reviewed-by: Michaël Paquier, Álvaro Herrera
Discussion: https://postgr.es/m/CAKcux6=nTz9KSfTr_6Z2mpzLJ_09JN-rK6=dWic6gGyTSWueyQ@mail.gmail.com