tests: Avoid sed error on Mac OS X when dealing with binary input
Reported by Ramsey Dow in:
<http://lists.gnu.org/archive/html/bug-gettext/2014-07/msg00002.html>.
* msgmerge-compendium-6: Run sed with LC_ALL=C, to avoid 'illegal
byte sequence' error on Mac OS X.
* format-sh-1: Likewise.
