tests: remove last uses of C_LOCALE_OUTPUT

tests: remove last uses of C_LOCALE_OUTPUT

Remove the last uses of the C_LOCALE_OUTPUT prerequisite as well as
the prerequisite itself. This is a follow-up to d162b25f956 (tests:
remove support for GIT_TEST_GETTEXT_POISON, 2021-01-20), as well as
the preceding commit where we removed the simpler uses of
C_LOCALE_OUTPUT.

Here I'm slightly refactoring a test added in 21e5ad50fc5 (safecrlf:
Add mechanism to warn about irreversible crlf conversions,
2008-02-06), as well as getting rid of another "test_have_prereq
C_LOCALE_OUTPUT" use.

I'm not leaving the prerequisite itself in place for in-flight changes
as there currently are none that introduce new tests that rely on it,
and because C_LOCALE_OUTPUT is currently a noop on the master branch
we likely won't have any new submissions that use it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>