block: precompute ConsensusData hash on UnmarshalJSON

That's what we usually do for block or transaction hashes.

pkg/core/block/block.go

@@ -227,5 +227,8 @@ func (b *Block) UnmarshalJSON(data []byte) error {
 	b.Base = *base
 	b.Transactions = auxb.Transactions
 	b.ConsensusData = auxb.ConsensusData
+	// Some tests rely on hash presence and we're usually precomputing
+	// other hashes upon deserialization.
+	_ = b.ConsensusData.Hash()
 	return nil
 }