message: Add test for make_integrity_err
diff: @@ -200,6 +200,15 @@ mod test {
  }
  }
  + #[test]
  + fn make_integrity_err_works() {
  + let err = make_integrity_err();
  + match err {
  + VmError::IntegrityErr { .. } => {}
  + _ => panic!("Unexpected error"),
  + }
  + }
  +
  #[test]
  fn make_region_length_too_big_works() {
  let err = make_region_length_too_big(50, 20);
  