pgsql: Add some tests for encoding conversion in COPY TO/FROM

Add some tests for encoding conversion in COPY TO/FROM

This adds a couple of tests to trigger encoding conversion when input
and server encodings do not match in COPY FROM/TO, or need_transcoding
set to true in the COPY state data. These tests rely on UTF8 <-> LATIN1
for the valid cases as LATIN1 accepts any bytes, and UTF8 <-> EUC_JP for
some of the invalid cases where a character cannot be understood,
causing a conversion failure.

Both ENCODING and client_encoding are covered. Test suggested by Andres
Freund.

Author: Sutou Kouhei
Discussion: https://postgr.es/m/20240206222445.hzq22pb2nye7rm67@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3ad8b840ce8b1d7279f2d0d5fb7d346c0a6a3e8d

Modified Files
--------------
src/test/regress/expected/copyencoding.out | 46 ++++++++++++++++++++++++
src/test/regress/expected/copyencoding_1.out | 8 +++++
src/test/regress/parallel_schedule | 2 +-
src/test/regress/sql/copyencoding.sql | 53 ++++++++++++++++++++++++++++
4 files changed, 108 insertions(+), 1 deletion(-)