testsuite: fix du testsuite to not use changing directories

diff --git a/testsuite/du/du-s-works b/testsuite/du/du-s-works
index f0b3bf0..ae97077 100644
--- a/testsuite/du/du-s-works
+++ b/testsuite/du/du-s-works
@@ -1,4 +1,4 @@
-[ -n "$d" ] || d=..
+d=/bin
 du -s "$d" > logfile.gnu
 busybox du -s "$d" > logfile.bb
 cmp logfile.gnu logfile.bb