Reproduce test fixture xprv (with and without path)

I need to figure out whether this uses an HD path or if it's straight to xprv, etc.

```txt
xprv9s21ZrQH143K2V4oox4M8Zmhi2Fjx5XK4Lf7GKRvPSgydU3mjZuKGCTg7UPiBUD7ydVPvSLtg9hjp7MQTYsW67rZHAXeccqYqrsx8LcXnyd
```

My guess, since it's part of the BIP-39, not the BIP-44, is that it's straight to xprv, no path.