Block the running of a known hanging regression test on OS X. Partial fix for bz#344416, and related to BZ#216837.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15417 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/helgrind/tests/pth_cond_destroy_busy.vgtest b/helgrind/tests/pth_cond_destroy_busy.vgtest
index c2e380e..fe7ac24 100644
--- a/helgrind/tests/pth_cond_destroy_busy.vgtest
+++ b/helgrind/tests/pth_cond_destroy_busy.vgtest
@@ -1 +1,2 @@
+prereq: ! `../../tests/os_test darwin`
 prog: ../../drd/tests/pth_cond_destroy_busy