pgsql: amcheck: Fix snapshot usage in bt_index_parent_check

amcheck: Fix snapshot usage in bt_index_parent_check

We were using SnapshotAny to do some index checks, but that's wrong and
causes spurious errors when used on indexes created by CREATE INDEX
CONCURRENTLY.  Fix it to use an MVCC snapshot, and add a test for it.

This problem came in with commit 5ae2087202af, which introduced
uniqueness check.  Backpatch to 17.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Backpatch-through: 17
Discussion: https://postgr.es/m/CANtu0ojmVd27fEhfpST7RG2KZvwkX=dMyKUqg0KM87FkOSdz8Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6bd469d26aca6ea413b35bfcb611dfa3a8f5ea45

Modified Files
--------------
contrib/amcheck/t/002_cic.pl    | 23 +++++++++++
contrib/amcheck/verify_nbtree.c | 86 +++++++++++++++++------------------------
doc/src/sgml/amcheck.sgml       |  2 +-
3 files changed, 60 insertions(+), 51 deletions(-)