message: Check ivk in test vectors
diff: @@ -1193,6 +1193,14 @@ mod tests {
  assert_eq!(xfvk.dk.0, tv.dk);
  assert_eq!(xfvk.chain_code.0, tv.c);
  + xfvk.fvk
  + .vk
  + .ivk()
  + .into_repr()
  + .write_le(&mut buf[..])
  + .unwrap();
  + assert_eq!(buf, tv.ivk);
  +
  let mut ser = vec![];
  xfvk.write(&mut ser).unwrap();
  assert_eq!(&ser[..], &tv.xfvk[..]);
  