Optimistic sync spec tests (v1.2.0) (#3564)

## Issue Addressed

Implements new optimistic sync test format from https://github.com/ethereum/consensus-specs/pull/2982.

## Proposed Changes

- Add parsing and runner support for the new test format.
- Extend the mock EL with a set of canned responses keyed by block hash. Although this doubles up on some of the existing functionality I think it's really nice to use compared to the `preloaded_responses` or static responses. I think we could write novel new opt sync tests using these primtives much more easily than the previous ones. Forks are natively supported, and different responses to `forkchoiceUpdated` and `newPayload` are also straight-forward.

## Additional Info

Blocked on merge of the spec PR and release of new test vectors.

testing/ef_tests/src/cases/operations.rs

@@ -117,6 +117,11 @@ impl<E: EthSpec> Operation<E> for Deposit {
         ssz_decode_file(path)
     }
 
+    fn is_enabled_for_fork(_: ForkName) -> bool {
+        // Some deposit tests require signature verification but are not marked as such.
+        cfg!(not(feature = "fake_crypto"))
+    }
+
     fn apply_to(
         &self,
         state: &mut BeaconState<E>,