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.

beacon_node/beacon_chain/src/test_utils.rs

@@ -46,8 +46,6 @@ use types::{
 
 // 4th September 2019
 pub const HARNESS_GENESIS_TIME: u64 = 1_567_552_690;
-// This parameter is required by a builder but not used because we use the `TestingSlotClock`.
-pub const HARNESS_SLOT_TIME: Duration = Duration::from_secs(1);
 // Environment variable to read if `fork_from_env` feature is enabled.
 const FORK_NAME_ENV_VAR: &str = "FORK_NAME";
 
@@ -182,6 +180,27 @@ impl<E: EthSpec> Builder<EphemeralHarnessType<E>> {
         self.store = Some(store);
         self.store_mutator(Box::new(mutator))
     }
+
+    /// Create a new ephemeral store that uses the specified `genesis_state`.
+    pub fn genesis_state_ephemeral_store(mut self, genesis_state: BeaconState<E>) -> Self {
+        let spec = self.spec.as_ref().expect("cannot build without spec");
+
+        let store = Arc::new(
+            HotColdDB::open_ephemeral(
+                self.store_config.clone().unwrap_or_default(),
+                spec.clone(),
+                self.log.clone(),
+            )
+            .unwrap(),
+        );
+        let mutator = move |builder: BeaconChainBuilder<_>| {
+            builder
+                .genesis_state(genesis_state)
+                .expect("should build state using recent genesis")
+        };
+        self.store = Some(store);
+        self.store_mutator(Box::new(mutator))
+    }
 }
 
 impl<E: EthSpec> Builder<DiskHarnessType<E>> {
@@ -297,6 +316,7 @@ where
 
         let log = test_logger();
         let spec = self.spec.expect("cannot build without spec");
+        let seconds_per_slot = spec.seconds_per_slot;
         let validator_keypairs = self
             .validator_keypairs
             .expect("cannot build without validator keypairs");
@@ -331,7 +351,7 @@ where
         // Initialize the slot clock only if it hasn't already been initialized.
         builder = if builder.get_slot_clock().is_none() {
             builder
-                .testing_slot_clock(HARNESS_SLOT_TIME)
+                .testing_slot_clock(Duration::from_secs(seconds_per_slot))
                 .expect("should configure testing slot clock")
         } else {
             builder