message: Add test for +=
diff: @@ -203,10 +203,16 @@ mod test {
  let a = Uint128(12345);
  let b = Uint128(23456);
  + // test + and - for valid values
  assert_eq!(a + b, Uint128(35801));
  assert_eq!((b - a).unwrap(), Uint128(11111));
  - // error result on underflow
  + // test +=
  + let mut c = Uint128(300000);
  + c += b;
  + assert_eq!(c, Uint128(323456));
  +
  + // error result on underflow (- would produce negative result)
  let underflow = a - b;
  match underflow {
  Ok(_) => panic!("should error"),
  