ThreadNet tests should involve translating transactions across eras

At the moment, the Shelley `ThreadNet` tests do not generate random transactions, for various reasons.

In particular, this means we are not exercising the translation of transactions across eras. This happens in two ways:

* A tx might arrive at the node shortly after its local chain made the era transition. This would require the tx to be translated when added to the mempool.

* A tx might enter the mempool shortly before the local chain make the era transition. This would require the tx to be translated when the mempool is revalidated against the new ledger state.