Fix the chance of tests being discarded for MR property specs

I spent ages trying to get rid of _all_ the discards. I found that even `Gen.choose` is going to fail every so often, but because of `private` methods/classes I couldn't do much about it. :(

/cc @etorreborre You might be interested
posNum doesn't include zero? This surprises me, but if that is normal behavior we should match.