Run ByronSpec ledger with BFT

input-output-hk/ouroboros-network#1425 introduced a dual ledger running the Byron implementation alongside the Byron spec. We can't currently run the byron spec by itself, because I didn't implement the consensus plumbing for it (which is handled in the dual ledger by the Byron _implementation_ instead). We could however run our consensus tests with the Byron spec only; it would not be much work, a handful of small classes to implement, the vast majority of the work is already done. It would have to run with the BFT protocol, rather than PBFT, since it supports mock crypto, but that's no big deal.

It would be useful to define this so that we can isolate any problems with the spec from problems we detect from running the dual ledger.