chanmgr: add a test for AbstractChannel::duration_unused

            let chan = r1.unwrap();

            assert_eq!(chan.ident(), &ed);

            assert!(chan.is_usable());

            // In theory, time could pass here, so we can't just use

            // "assert_eq!(dur_unused, dur_unused2)".

            let dur_unused = Channel::duration_unused(&chan);

            let dur_unused_2 = AbstractChannel::duration_unused(&chan);

            let dur_unused_3 = Channel::duration_unused(&chan);

            assert!(dur_unused.unwrap() <= dur_unused_2.unwrap());

            assert!(dur_unused_2.unwrap() <= dur_unused_3.unwrap());

            r2.unwrap();

            Ok(())

        })