shellsortorder tests should also test the update to 'i'

#158 adds tests of `shellsortorder()`, but only of the `y` output, not of `i`. I don't really understand this function yet, and am only flying by for now, so earmarking this for future work.

```r
x = as.integer64(1:3)
y = sample(x)
y
# integer64
# [1] 1 3 2
i = seq_along(y)
shellsortorder(y, i)
# [1] 0
y
# integer64
# [1] 1 2 3
i
# [1] 1 3 2
```