Add fork choice EF tests (#2737)

## Issue Addressed

Resolves #2545

## Proposed Changes

Adds the long-overdue EF tests for fork choice. Although we had pretty good coverage via other implementations that closely followed our approach, it is nonetheless important for us to implement these tests too.

During testing I found that we were using a hard-coded `SAFE_SLOTS_TO_UPDATE_JUSTIFIED` value rather than one from the `ChainSpec`. This caused a failure during a minimal preset test. This doesn't represent a risk to mainnet or testnets, since the hard-coded value matched the mainnet preset.

## Failing Cases

There is one failing case which is presently marked as `SkippedKnownFailure`:

```
case 4 ("new_finalized_slot_is_justified_checkpoint_ancestor") from /home/paul/development/lighthouse/testing/ef_tests/consensus-spec-tests/tests/minimal/phase0/fork_choice/on_block/pyspec_tests/new_finalized_slot_is_justified_checkpoint_ancestor failed with NotEqual:
head check failed: Got Head { slot: Slot(40), root: 0x9183dbaed4191a862bd307d476e687277fc08469fc38618699863333487703e7 } | Expected Head { slot: Slot(24), root: 0x105b49b51bf7103c182aa58860b039550a89c05a4675992e2af703bd02c84570 }
```

This failure is due to #2741. It's not a particularly high priority issue at the moment, so we fix it after merging this PR.

testing/ef_tests/src/handler.rs

@@ -421,6 +421,58 @@ impl<E: EthSpec + TypeName> Handler for FinalityHandler<E> {
     }
 }
 
+#[derive(Derivative)]
+#[derivative(Default(bound = ""))]
+pub struct ForkChoiceGetHeadHandler<E>(PhantomData<E>);
+
+impl<E: EthSpec + TypeName> Handler for ForkChoiceGetHeadHandler<E> {
+    type Case = cases::ForkChoiceTest<E>;
+
+    fn config_name() -> &'static str {
+        E::name()
+    }
+
+    fn runner_name() -> &'static str {
+        "fork_choice"
+    }
+
+    fn handler_name(&self) -> String {
+        "get_head".into()
+    }
+
+    fn is_enabled_for_fork(&self, _fork_name: ForkName) -> bool {
+        // These tests check block validity (which may include signatures) and there is no need to
+        // run them with fake crypto.
+        cfg!(not(feature = "fake_crypto"))
+    }
+}
+
+#[derive(Derivative)]
+#[derivative(Default(bound = ""))]
+pub struct ForkChoiceOnBlockHandler<E>(PhantomData<E>);
+
+impl<E: EthSpec + TypeName> Handler for ForkChoiceOnBlockHandler<E> {
+    type Case = cases::ForkChoiceTest<E>;
+
+    fn config_name() -> &'static str {
+        E::name()
+    }
+
+    fn runner_name() -> &'static str {
+        "fork_choice"
+    }
+
+    fn handler_name(&self) -> String {
+        "on_block".into()
+    }
+
+    fn is_enabled_for_fork(&self, _fork_name: ForkName) -> bool {
+        // These tests check block validity (which may include signatures) and there is no need to
+        // run them with fake crypto.
+        cfg!(not(feature = "fake_crypto"))
+    }
+}
+
 #[derive(Derivative)]
 #[derivative(Default(bound = ""))]
 pub struct GenesisValidityHandler<E>(PhantomData<E>);