Clean up date_part tests a bit

Peter Eisentraut <peter@eisentraut.org>

Commit: efcc7572f532ea564fedc6359c2df43045ee7908
Author: Peter Eisentraut <peter@eisentraut.org>
Date: 2021-03-29T15:53:30Z
Releases: 14.0
Clean up date_part tests a bit

Some tests for timestamp and timestamptz were in the date.sql test
file.  Move them to their appropriate files, or drop tests cases that
were already present there.

Files