pgsql: Add LP_DEAD deletion of a posting list tuple test.

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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/52b164c5a00095a34685e66bf64b009578b9cfda

Modified Files
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src/test/regress/expected/btree_index.out | 13 ++++++++++---
src/test/regress/sql/btree_index.sql      | 14 +++++++++++---
2 files changed, 21 insertions(+), 6 deletions(-)