Add unit tests for orderby that uses minimal data

@ian29 @camilleanne when you have a chance give this a read and let me know if this approach makes sense for other FX as well.

The tests are currently failing the *preserves order of tied features* assertion because the fx currently reorders tied features.

- [x] Fix orderby to preserve order of ties
- [x] Review approach by @ian29 @camilleanne 
- [ ] Mergetown and write more tests!