shellsortorder tests should also test the update to 'i'

[MichaelChirico]

#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.

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