Add BeaconChainHarness::builder (#2707)

## Issue Addressed

NA

## Proposed Changes

This PR is near-identical to https://github.com/sigp/lighthouse/pull/2652, however it is to be merged into `unstable` instead of `merge-f2f`. Please see that PR for reasoning.

I'm making this duplicate PR to merge to `unstable` in an effort to shrink the diff between `unstable` and `merge-f2f` by doing smaller, lead-up PRs.

## Additional Info

NA

beacon_node/network/src/beacon_processor/tests.rs

@@ -23,8 +23,8 @@ use std::time::Duration;
 use tokio::runtime::Runtime;
 use tokio::sync::mpsc;
 use types::{
-    test_utils::generate_deterministic_keypairs, Attestation, AttesterSlashing, EthSpec,
-    MainnetEthSpec, ProposerSlashing, SignedBeaconBlock, SignedVoluntaryExit, SubnetId,
+    Attestation, AttesterSlashing, EthSpec, MainnetEthSpec, ProposerSlashing, SignedBeaconBlock,
+    SignedVoluntaryExit, SubnetId,
 };
 
 type E = MainnetEthSpec;
@@ -75,11 +75,11 @@ impl TestRig {
         let mut spec = E::default_spec();
         spec.shard_committee_period = 2;
 
-        let harness = BeaconChainHarness::new(
-            MainnetEthSpec,
-            Some(spec),
-            generate_deterministic_keypairs(VALIDATOR_COUNT),
-        );
+        let harness = BeaconChainHarness::builder(MainnetEthSpec)
+            .spec(spec)
+            .deterministic_keypairs(VALIDATOR_COUNT)
+            .fresh_ephemeral_store()
+            .build();
 
         harness.advance_slot();
 

beacon_node/network/src/service/tests.rs

@@ -9,9 +9,8 @@ mod tests {
     use sloggers::{null::NullLoggerBuilder, Build};
     use std::str::FromStr;
     use std::sync::Arc;
-    use store::config::StoreConfig;
     use tokio::runtime::Runtime;
-    use types::{test_utils::generate_deterministic_keypairs, MinimalEthSpec};
+    use types::MinimalEthSpec;
 
     fn get_logger(actual_log: bool) -> Logger {
         if actual_log {
@@ -35,13 +34,12 @@ mod tests {
     fn test_dht_persistence() {
         let log = get_logger(false);
 
-        let beacon_chain = BeaconChainHarness::new_with_store_config(
-            MinimalEthSpec,
-            None,
-            generate_deterministic_keypairs(8),
-            StoreConfig::default(),
-        )
-        .chain;
+        let beacon_chain = BeaconChainHarness::builder(MinimalEthSpec)
+            .default_spec()
+            .deterministic_keypairs(8)
+            .fresh_ephemeral_store()
+            .build()
+            .chain;
 
         let store = beacon_chain.store.clone();