tests: Ensure gettext strings are loaded in UTF-8 in gdatetime.c

See the previous commit. By convention, GLib assumes strings loaded from
gettext are always in UTF-8, but we do need to tell gettext this. In
most other tests, it doesn’t matter; but in the gdatetime test, we test
re-encoding month names from EUC-JP, so we need to ensure the
translations start in UTF-8 correctly.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793578