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/src/cli.rs

@@ -1509,6 +1509,19 @@ pub fn cli_app() -> Command {
                 .help_heading(FLAG_HEADER)
                 .display_order(0)
         )
+        .arg(
+            Arg::new("sync-tolerance-epochs")
+                .long("sync-tolerance-epochs")
+                .help("Overrides the default SYNC_TOLERANCE_EPOCHS. This flag is not intended \
+                    for production and MUST only be used in TESTING only. This is primarily used \
+                    for testing range sync, to prevent the node from producing a block before the \
+                    node is synced with the network which may result in the node getting \
+                    disconnected from peers immediately.")
+                .hide(true)
+                .requires("enable_http")
+                .action(ArgAction::Set)
+                .display_order(0)
+        )
         .arg(
             Arg::new("gui")
                 .long("gui")

beacon_node/src/config.rs

@@ -8,7 +8,7 @@ use beacon_chain::graffiti_calculator::GraffitiOrigin;
 use beacon_chain::TrustedSetup;
 use clap::{parser::ValueSource, ArgMatches, Id};
 use clap_utils::flags::DISABLE_MALLOC_TUNING_FLAG;
-use clap_utils::{parse_flag, parse_required};
+use clap_utils::{parse_flag, parse_optional, parse_required};
 use client::{ClientConfig, ClientGenesis};
 use directory::{DEFAULT_BEACON_NODE_DIR, DEFAULT_NETWORK_DIR, DEFAULT_ROOT_DIR};
 use environment::RuntimeContext;
@@ -174,6 +174,9 @@ pub fn get_config<E: EthSpec>(
 
         client_config.http_api.duplicate_block_status_code =
             parse_required(cli_args, "http-duplicate-block-status")?;
+
+        client_config.http_api.sync_tolerance_epochs =
+            parse_optional(cli_args, "sync-tolerance-epochs")?;
     }
 
     if cli_args.get_flag("light-client-server") {