Merge RealPBFT and DualPBFT tests

At the moment, the Byron integration is tested in two different tests: the RealPBFT and the DualPBFT tests. RealPBFT tests delegation certificates and (after input-output-hk/ouroboros-network#1514) updates, and DualPBFT tests regular transactions. Although that means that between the two of them we test the entire ledger, it means we could potentially miss any kind of subtle interactions between these two, and so it would be better to combine these two tests.

This isn't trivial, because delegation certificates and update proposals require node restarts. At the moment, the tests decide on a static "restart schedule" before the test is run, but the transaction generator is _not_ static, and can't be: the transaction generator depends on the state of the chain, which depends on blocks being sent around, chain selection, etc.