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>
Forwarded: https://github.com/ostreedev/ostree/pull/1188
Applied-upstream: 2017.12, commit:223c940b46a4bb335665df7436566b73cdf0effd

Gbp-Pq: Topic 2017.12
Gbp-Pq: Name tests-Explicitly-unset-LANGUAGE-after-setting-LC_ALL.patch