Use fork data from the state rather than derived from the params

This change is more in line with how this functionality will ultimately be used
so we want to route our query through the `BeaconState`.

Author: ralexstokes

tests/beacon/test_helpers.py

@@ -918,18 +918,17 @@ def test_verify_slashable_vote_data_signature(num_validators,
     # touch disjoint subsets of the provided params
     messages = _create_slashable_vote_data_messages(sample_slashable_vote_data_params)
 
-    fork_data = ForkData(**sample_fork_data_params)
     valid_params = _correct_slashable_vote_data_params(
         num_validators,
         sample_slashable_vote_data_params,
         messages,
         privkeys,
-        fork_data,
+        state.fork_data,
     )
     valid_votes = SlashableVoteData(**valid_params)
     assert verify_slashable_vote_data_signature(state, valid_votes)
 
-    invalid_params = _corrupt_signature(valid_params, fork_data)
+    invalid_params = _corrupt_signature(valid_params, state.fork_data)
     invalid_votes = SlashableVoteData(**invalid_params)
     assert not verify_slashable_vote_data_signature(state, invalid_votes)
 
@@ -992,16 +991,15 @@ def test_verify_slashable_vote_data(num_validators,
     # touch disjoint subsets of the provided params
     messages = _create_slashable_vote_data_messages(sample_slashable_vote_data_params)
 
-    fork_data = ForkData(**sample_fork_data_params)
     params = _correct_slashable_vote_data_params(
         num_validators,
         sample_slashable_vote_data_params,
         messages,
         privkeys,
-        fork_data,
+        state.fork_data,
     )
     if needs_fork_data:
-        params = param_mapper(params, fork_data)
+        params = param_mapper(params, state.fork_data)
     else:
         params = param_mapper(params)
     _run_verify_slashable_vote(params, state, max_casper_votes, should_succeed)