Remove nondeterministic tests?

[newpavlov]

Right now we have a bunch of tests which use RNG seeded from system entropy. In general, nondeterministic tests are not great since they can result in unreproducible test failures. I suggest we replace them with RNG seeded from a fixed seed.

[tarcieri]

Yeah, I think nondeterminism only makes sense in the context of something like property-based testing, where it can provide you a seed to reproduce the error, and also keep a history of previously problematic seeds for regression testing