Check correctness of mod and rem

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.