makeTagishRecord = (tag = 'Remotable', proto = undefined) => {
  return Object.create(proto === undefined ? Object.prototype : proto, {
    [PASS_STYLE]: { value: 'remotable' },
    [Symbol.toStringTag]: { value: tag },
  });
}
For testing purposes, makes a *non-frozen* TagRecord-like object with
non-enumerable PASS_STYLE and Symbol.toStringTag properties.
A valid Remotable must inherit from a valid TagRecord.
- Before stabilize/suppressTrapping, a valid TagRecord must be frozen.
- After stabilize/suppressTrapping, a valid TagRecord must also be
  stable/non-trapping, for example, because it was hardened.

@param {string} [tag]
@param {object|null} [proto]
@returns {{ [PASS_STYLE]: 'remotable', [Symbol.toStringTag]: string }}
@see https://github.com/endojs/endo/blob/master/packages/ses/docs/preparing-for-stabilize.md