tests: Explicitly unset LANGUAGE after setting LC_ALL

tests: Explicitly unset LANGUAGE after setting LC_ALL

As a GNU extension, LANGUAGE takes precedence over LC_ALL for
gettext(3) whenever the locale is not C, causing tests that grep for
specific English strings to fail when run in non-English locales.
The upstream glibc proposal for C.UTF-8 would give C.UTF-8 the same
special case as C here, but the implementation in Debian does not
currently have this, so we have to unset LANGUAGE too.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Closes: #1188
Approved by: jlebon