pgsql: Add C test function for pg_locale.h APIs.

Add C test function for pg_locale.h APIs.

Test the API independently to account for fallback paths that aren't
adequately tested from SQL.

The backport to 18 also tests the previously-supported behavior where
a size of -1 meant that the string was NUL-terminated. That behavior
was later removed in 19.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/v36ssaygf7grb3qzfsjhtdzi7kqd45ds56nyuf7gi5qjml4qbb@ezmfqzmhlrs2
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ed37fefedb3f81cddca576bb256f162d5c036ab1

Modified Files
--------------
src/test/regress/expected/misc_functions.out | 40 +++++++
src/test/regress/regress.c | 150 +++++++++++++++++++++++++++
src/test/regress/sql/misc_functions.sql | 25 +++++
3 files changed, 215 insertions(+)