Add test for killpg

Author: DanSnow

test/sys/test_signal.rs

@@ -9,6 +9,11 @@ fn test_kill_none() {
     kill(getpid(), None).expect("Should be able to send signal to myself.");
 }
 
+#[test]
+fn test_killpg_none() {
+    killpg(getpgrp(), None).expect("Should be able to send signal to my process group.");
+}
+
 #[test]
 fn test_old_sigaction_flags() {
     extern "C" fn handler(_: ::libc::c_int) {}