message: Add an explanatory comment.
diff: @@ -227,11 +227,15 @@ where
  validator_id: ValidatorId,
  message: Message<HighwayMessage<Ctx>>,
  ) -> Result<Vec<HighwayMessage<Ctx>>, TestRunError<Ctx>> {
  + // Using the same `recipient` instance in the whole method body is not possible due to compiler
  + // complaining about using `self` as mutable and immutable.
  + {
  let recipient = self
  .virtual_net
  .get_validator_mut(&validator_id)
  .ok_or_else(|| TestRunError::MissingValidator(validator_id))?;
  recipient.push_messages_received(vec![message.clone()]);
  + }
  let messages = {
  let sender_id = message.sender;
  