Make the markup-parse test independent of the locale

I added a setlocale call, because we need it for Unicode to
come out right; but I forgot to fix the locale, so we now
fail when comparing error messages to the expected (English)
result. Correct this by setting LANG explicitly to en_US.utf-8.

https://bugzilla.gnome.org/show_bug.cgi?id=669285

glib/tests/markup-parse.c

@@ -281,6 +281,7 @@ main (int argc, char *argv[])
   const gchar *name;
   gchar *path;
 
+  g_setenv ("LANG", "en_US.utf-8", TRUE);
   setlocale (LC_ALL, "");
 
   g_test_init (&argc, &argv, NULL);