fix sb-posix/posix-tests.lisp failed on Japanese Mac OS X

## posix-tests.lisp.patch
diff --git a/contrib/sb-posix/posix-tests.lisp b/contrib/sb-posix/posix-tests.lisp
index ec5a49c..444b3b0 100644
--- a/contrib/sb-posix/posix-tests.lisp
+++ b/contrib/sb-posix/posix-tests.lisp
@@ -3,6 +3,9 @@

 (in-package "SB-POSIX-TESTS")

+#+darwin(setf sb-alien::*default-c-string-external-format* :utf-8)
+#+darwin(setf sb-impl::*default-external-format* :utf-8)
+
 (defvar *test-directory*
   (ensure-directories-exist
    (merge-pathnames (make-pathname :directory '(:relative "test-output"))
@@ -547,7 +550,6 @@
       (sb-posix:closedir dir)))
   t)

-#-darwin
 (deftest readdir/dirent-name
     (let ((dir (sb-posix:opendir *current-directory*)))
       (unwind-protect