Avoid trying to fetch metapage of an SPGist partitioned index.

This is necessary when spgcanreturn() is invoked on a partitioned
index, and the failure might be reachable in other scenarios as
well.  The rest of what spgGetCache() does is perfectly sensible
for a partitioned index, so we should allow it to go through.

I think the main takeaway from this is that we lack sufficient test
coverage for non-btree partitioned indexes.  Therefore, I added
simple test cases for brin and gin as well as spgist (hash and
gist AMs were covered already in indexing.sql).

Per bug #18256 from Alexander Lakhin.  Although the known test case
only fails since v16 (3c569049b), I've got no faith at all that there
aren't other ways to reach this problem; so back-patch to all
supported branches.

Discussion: https://postgr.es/m/18256-0b0e1b6e4a620f1b@postgresql.org

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