static serializeOptions(opts = {}) {
    if (!optionsCache.has(opts)) {
      const serialized = serializeJavascript(opts, {
        unsafe: true, // this means we don't care about XSS
        ignoreFunction: true // do not serialize functions
      });
      optionsCache.set(opts, serialized);
      /* istanbul ignore next */
      debug(
        'serializeOptions(): serialized options %O to: %s',
        opts,
        serialized
      );
    }
    return optionsCache.get(opts);
  }
Given Mocha options object `opts`, serialize into a format suitable for
transmission over IPC.

@param {Options} [opts] - Mocha options
@private
@returns {string} Serialized options