QuickCheck properties for IntSet

[Don Stewart]

dbenbenn:
> As I recall, IntSet overrides the number of times to check each QC
> property, to 500 or 5000?  Removing the override, or putting it down
> at 10 or 50, makes the checks run much faster.

Yeah, I think we can do quite well by having even only 10 tests, as
long as we use a new random seed each time. Given enough nightly
builds we'll end up testing more than with the fixed seed/1000 run.

-- Don