tests: don't let the LANGUAGE envvar perturb tests

* tests/envvar-check (vars): Add LANGUAGE to the list of envvars
to unset.  At least in glibc (as an extension to POSIX), its value
actually trumps LC_ALL:

  $ LC_ALL=es_ES LANGUAGE=fr_FR.UTF-8 /bin/cat no-such
  /bin/cat: no-such: Aucun fichier ou dossier de ce type

but only when the default locale is not C:

  $ LC_ALL=C LANGUAGE=fr_FR.UTF-8 /bin/cat no-such
  /bin/cat: no-such: No such file or directory

Prompted by a report from Mads Kiilerich.

tests/envvar-check

@@ -31,6 +31,7 @@ vars='
   COLUMNS
   DF_BLOCK_SIZE
   DU_BLOCK_SIZE
+  LANGUAGE
   LS_BLOCK_SIZE
   LS_COLORS
   POSIXLY_CORRECT