Add two tests for send and sync

To prevent future regressions as the library evolves

src/ipv6.rs

@@ -389,4 +389,16 @@ mod test {
         let expected = Ipv6Network::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1), 128).unwrap();
         assert_eq!(net, expected);
     }
+
+    #[test]
+    fn test_send() {
+        fn assert_send<T: Send>() {}
+        assert_send::<Ipv6Network>();
+    }
+
+    #[test]
+    fn test_sync() {
+        fn assert_sync<T: Sync>() {}
+        assert_sync::<Ipv6Network>();
+    }
 }