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
```

lighthouse/tests/beacon_node.rs

@@ -2543,6 +2543,17 @@ fn light_client_http_server_disabled() {
         });
 }
 
+#[test]
+fn sync_tolerance_epochs() {
+    CommandLineTest::new()
+        .flag("http", None)
+        .flag("sync-tolerance-epochs", Some("0"))
+        .run_with_zero_port()
+        .with_config(|config| {
+            assert_eq!(config.http_api.sync_tolerance_epochs, Some(0));
+        });
+}
+
 #[test]
 fn gui_flag() {
     CommandLineTest::new()