Add more tests for VACUUM skips with partitioned tables

Add more tests for VACUUM skips with partitioned tables

A VACUUM or ANALYZE command listing directly a partitioned table expands
it to its partitions, causing all elements of a tree to be processed
with individual ownership checks done.  This results in different
relation skips depending on the ownership policy of a tree, which may
not be consistent for a partition tree.  This commit adds more tests to
ensure that any future refactoring allows to keep a consistent behavior,
or at least that any changes done are easily identified and checked.
The current behavior of VACUUM with partitioned tables is present since
10.

Author: Nathan Bossart
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/DC186201-B01F-4A66-9EC4-F855A957C1F9@amazon.com