make sure test execution is reproducible

[pveber]

It seems to me that ounit runs tests in parallel, which makes our use of Random functions not reproducible. This is not great for (cross-platform) debugging, and should be fixed.

• ensure tests are reproducible and controlled by a unique seed
• add a command line option to set the seed

[agarwal]

Perhaps this would be facilitated by using Core's Quickcheck, as suggested in #20. It appears to have well defined support for this.