Expand functionality of the unittests/finality_test_cluster

[systemzax]

When producing a block with the cluster, the signed_block_ptr should be returned, as well as the trace containing the on_block action data and receipt. Alternatively, could return the action receipt digest for the on_block action.

In addition, the cluster should expose a public function change_finalizer_policy(), which changes the bls public key of node2, and calls the setfinalizer action.