pgsql: Add tests for date_part of epoch near upper bound of timestamp r

Add tests for date_part of epoch near upper bound of timestamp range

This exercises a special case in the implementations of
date_part('epoch', timestamp[tz]) that was previously not tested.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6131ffc43ff3d2f566e93f017e56a09e4e717318

Modified Files
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src/test/regress/expected/timestamp.out   | 7 +++++++
src/test/regress/expected/timestamptz.out | 7 +++++++
src/test/regress/sql/timestamp.sql        | 3 +++
src/test/regress/sql/timestamptz.sql      | 3 +++
4 files changed, 20 insertions(+)