Add testing for beacon node and validator client CLI flags (#2311)

## Issue Addressed

N/A

## Proposed Changes

Add unit tests for the various CLI flags associated with the beacon node and validator client. These changes require the addition of two new flags: `dump-config` and `immediate-shutdown`.

## Additional Info

Both `dump-config` and `immediate-shutdown` are marked as hidden since they should only be used in testing and other advanced use cases.
**Note:** This requires changing `main.rs` so that the flags can adjust the program behavior as necessary.

Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/beacon_chain/src/test_utils.rs

@@ -25,6 +25,7 @@ use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
 use std::time::Duration;
 use store::{config::StoreConfig, BlockReplay, HotColdDB, ItemStore, LevelDB, MemoryStore};
+use task_executor::ShutdownReason;
 use tempfile::{tempdir, TempDir};
 use tree_hash::TreeHash;
 use types::{
@@ -115,7 +116,7 @@ pub struct BeaconChainHarness<T: BeaconChainTypes> {
     pub chain: BeaconChain<T>,
     pub spec: ChainSpec,
     pub data_dir: TempDir,
-    pub shutdown_receiver: Receiver<&'static str>,
+    pub shutdown_receiver: Receiver<ShutdownReason>,
 
     pub rng: Mutex<StdRng>,
 }