Verify drd/tests/pth_detached stderr output instead of the stdout output.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11933 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/drd/tests/pth_detached.vgtest b/drd/tests/pth_detached.vgtest
index f78b9cc..e91ae8e 100644
--- a/drd/tests/pth_detached.vgtest
+++ b/drd/tests/pth_detached.vgtest
@@ -1,3 +1,4 @@
 prereq: ./supported_libpthread
 prog: pth_detached
 args: 1 1
+stdout_filter: ../../tests/filter_sink