More tests: Broadcast

[afck]

Add an adversary that sends random (but "valid looking") messages. The distributions should be general, but somewhat intelligent, so that they are more likely to uncover bugs.

Also, the original idea behind the ProposeAdversary was that the faulty nodes would act as if they were part of another network with the same proposer but a different proposed value. (Currently they're just all proposing, which in most cases will simply be ignored.) So the attacker should simulate a separate network with the same IDs and the same proposer but a different input value. It should then send in the real network the same messages that its nodes sent in the simulation.