gettext tests: test re-encoding with a UTF-8 msgid under Shell

gettext tests: test re-encoding with a UTF-8 msgid under Shell

A test that tests that calling gettext on a UTF-8 msgid works, and
that recoding the resulting string works too.

This test uses the --from-code=UTF-8 xgettext(1) argument introduced
in an earlier patch.

This patch only tests the shellscript portion of our gettext
interface. I can't get any of these tests to fail on any of the
gettext implementations I have around, even without the previous patch
to gettext.c. But having exhaustive tests in this area is good
regardless.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>