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