Add test flag to override SYNC_TOLERANCE_EPOCHS for range sync testing (#7030)

Related to #6880, an issue that's usually observed on local devnets with small number of nodes.

When testing range sync, I usually shutdown a node for some period of time and restart it again. However, if it's within `SYNC_TOLERANCE_EPOCHS` (8), Lighthouse would consider the node as synced, and if it may attempt to produce a block if requested by a validator - on a local devnet, nodes frequently produce blocks - when this happens, the node ends up producing a block that would revert finality and would get disconnected from peers immediately.

### Usage

Run Lighthouse BN with this flag to override:

```
--sync-tolerance--epoch 0
```

beacon_node/http_api/src/lib.rs

@@ -112,7 +112,7 @@ const API_PREFIX: &str = "eth";
 ///
 /// This helps prevent attacks where nodes can convince us that we're syncing some non-existent
 /// finalized head.
-const SYNC_TOLERANCE_EPOCHS: u64 = 8;
+const DEFAULT_SYNC_TOLERANCE_EPOCHS: u64 = 8;
 
 /// A custom type which allows for both unsecured and TLS-enabled HTTP servers.
 type HttpServer = (SocketAddr, Pin<Box<dyn Future<Output = ()> + Send>>);
@@ -156,6 +156,7 @@ pub struct Config {
     #[serde(with = "eth2::types::serde_status_code")]
     pub duplicate_block_status_code: StatusCode,
     pub target_peers: usize,
+    pub sync_tolerance_epochs: Option<u64>,
 }
 
 impl Default for Config {
@@ -171,6 +172,7 @@ impl Default for Config {
             enable_beacon_processor: true,
             duplicate_block_status_code: StatusCode::ACCEPTED,
             target_peers: 100,
+            sync_tolerance_epochs: None,
         }
     }
 }
@@ -459,7 +461,10 @@ pub fn serve<T: BeaconChainTypes>(
                                     )
                                 })?;
 
-                            let tolerance = SYNC_TOLERANCE_EPOCHS * T::EthSpec::slots_per_epoch();
+                            let sync_tolerance_epochs = config
+                                .sync_tolerance_epochs
+                                .unwrap_or(DEFAULT_SYNC_TOLERANCE_EPOCHS);
+                            let tolerance = sync_tolerance_epochs * T::EthSpec::slots_per_epoch();
 
                             if head_slot + tolerance >= current_slot {
                                 Ok(())