Add LP_DEAD deletion of a posting list tuple test.

Peter Geoghegan <pg@bowt.ie>

Commit: 52b164c5a00095a34685e66bf64b009578b9cfda
Author: Peter Geoghegan <pg@bowt.ie>
Date: 2020-04-28T23:12:56Z
Releases: 13.0
Add LP_DEAD deletion of a posting list tuple test.

Make sure that we have test coverage of the posting list tuple path
within _bt_xid_horizon().

Per off-list complaint from Andres Freund.

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