xfail-fast run-pass/native-dupe.rs

This test calls the task pin function which has side effects and causes
other tests to lock.

src/test/run-pass/native-dupe.rs

@@ -1,3 +1,6 @@
+// xfail-fast - Somehow causes check-fast to livelock?? Probably because we're
+// calling pin_task and that's having wierd side-effects.
+
 native "cdecl" mod rustrt1 = "rustrt" {
     fn pin_task();
 }