rsvg-test.c: Override the environment's locale with LC_ALL
rsvg-test.c: Override the environment's locale with LC_ALL

We use a made-up set of language preferences for tests with the
systemLanguage attribute.  Previously we were using the LANGUAGE
environment variable, which is a GNU extension.

Now we use LC_ALL, which is the "override everything" standard variable.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg