ats dict as a string suitable as test name.
// format_value() is provided by testharness.js,
// which also preserves sign for -0.
function format_dict(dict) {
  const props = [];
  for (let prop in dict) {
    props.push(`${prop}: ${format_value(dict[prop])}`);
  }
  return `{${props.join(', ')}}`;
}

function format_input(input) {
  if (input === "omitted argument") {
    return "";
  } else if (input === null || typeof input !== "object") {
    return format_value(input);
  }
  return format_dict(input);
