Test Duration::new panics on overflow

A `Duration` is created from a second and nanoseconds variable. The
documentation says: "This constructor will panic if the carry from the
nanoseconds overflows the seconds counter". This was, however, not tested
in the tests. I doubt the behavior will ever regress, but it is usually a
good idea to test all documented behavior.

src/libcore/tests/time.rs

@@ -11,6 +11,12 @@ fn creation() {
     assert_eq!(Duration::from_millis(4000), Duration::new(4, 0));
 }
 
+#[test]
+#[should_panic]
+fn new_overflow() {
+    let _ = Duration::new(::core::u64::MAX, 1_000_000_000);
+}
+
 #[test]
 fn secs() {
     assert_eq!(Duration::new(0, 0).as_secs(), 0);