how to unittest writeln in void

// Display an error in the terminal
void displayError(string errString, string line = "", long lineNumber = 0) {
  string message = "Error: ";
  if (lineNumber) {
    message ~= errString;
    message ~= " on line ";
    message ~= to!string(lineNumber);
  } else {
    message ~= errString;
  }
  if (line) message ~= "\n" ~ line;
  writeln(message);
  return;
}