Add posting list tuple amcheck test case.

Peter Geoghegan <pg@bowt.ie>

Commit: 20c6905dee43a8888090674cb3db9f953ae7f646
Author: Peter Geoghegan <pg@bowt.ie>
Date: 2020-05-04T18:23:44Z
Releases: 13.0
Add posting list tuple amcheck test case.

Add a test case to contrib/amcheck that creates coverage of code paths
that are used to verify posting list tuples (tuples created when
deduplication merges together existing tuples to avoid a leaf page
split).

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