Verify all Batchs published by the prior set

The new set publishing a `Batch` completes the handover protocol. The new set
should only publish a `Batch` once it believes the old set has completed all of
its on-external-chain activity, marking it honest and finite.

With the handover comes the acceptance of liability, hence the requirement for
all of the on-Serai-chain activity also needing verification. While most
activity would be verified in-real-time (upon ::Batch messages), the new set
will now explicitly verify the complete set of `Batch`s before beginning its
preprocess for its own `Batch` (the one accepting the handover).

coordinator/src/tests/tributary/chain.rs

@@ -54,7 +54,7 @@ pub fn new_spec<R: RngCore + CryptoRng>(
 
   let set_participants = keys
     .iter()
-    .map(|key| sr25519::Public((<Ristretto as Ciphersuite>::generator() * **key).to_bytes()))
+    .map(|key| (sr25519::Public((<Ristretto as Ciphersuite>::generator() * **key).to_bytes()), 1))
     .collect::<Vec<_>>();
 
   let res = TributarySpec::new(serai_block, start_time, set, set_participants);