message: Support unary operator(plus, minus) unit test
diff: @@ -272,6 +272,12 @@ mod tests {
  Number(Cow::Owned(BigDecimal::try_from(3.3).unwrap())).multiply(&mon(10)),
  Ok(mon(33))
  );
  + assert_eq!(num(2).unary_plus(), Ok(num(2)));
  + assert_eq!(mon(2).unary_plus(), Ok(mon(2)));
  + matches!(Null.unary_plus(), Ok(Null));
  + assert_eq!(num(1).unary_minus(), Ok(num(-1)));
  + assert_eq!(mon(1).unary_minus(), Ok(mon(-1)));
  + matches!(Null.unary_minus(), Ok(Null));
  }
  #[test]
  