Check correctness of mod and rem

[oxinabox]

We still use a explict seed for testing mod and rem. During #185 I removed it and those tests failed.

I have not investigated closely, it could be they are doing bad things with the domain. If so rather than random numbers we should hard-code sensible points. We do that in a lot of other tests. I think I in general like that, sensible arbitary numbers rather than seeded random numbers.