Add a new Coffeescript test that demonstrates RIPD-639.

Could you manage to bring that `RIPD_639_FIXED` flag over from your first attempt at this (#669)?

eg.
```
# Not sure where this would go
RIPD_639_FIXED = false
# Line 850
# I have no idea if that's the correct expected path
            alternatives: if RIPD_639_FIXED 
              then [ 
                 amount: "1/USD/G3"
                 paths: [[
                   "USD/GW1|$"
                   "USD/GW2|$"
                   "USD/GW3|$" 
                ]]
             ]
             else []
```

Look, ma! I'm writing Coffeescript!