[TSan][Darwin] Mark test unsupported

GitOrigin-RevId: 6703fe25b7c26742cb511ba0e6050173188dd2f1
diff --git a/test/tsan/on_exit.cpp b/test/tsan/on_exit.cpp
index 05e19ad..2dadd68 100644
--- a/test/tsan/on_exit.cpp
+++ b/test/tsan/on_exit.cpp
@@ -1,4 +1,8 @@
 // RUN: %clang_tsan -O1 %s -o %t && %deflake %run %t | FileCheck %s
+
+// on_exit() is not available on Darwin.
+// UNSUPPORTED: darwin
+
 #include "test.h"
 
 volatile long global;