Fix example tests on macosx

test_many failed to receive the second signal when they were sent
simultaneously.

examples/test_many.rs

@@ -10,7 +10,7 @@ fn main() {
  chan_signal::notify_on(&s1, Signal::HUP);
  chan_signal::notify_on(&s2, Signal::TERM);
  kill_this(Signal::HUP);
- kill_this(Signal::TERM);
  assert_eq!(r1.recv(), Some(Signal::HUP));
+ kill_this(Signal::TERM);
  assert_eq!(r2.recv(), Some(Signal::TERM));
 }