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/builder.rs

@@ -25,6 +25,7 @@ use std::marker::PhantomData;
 use std::sync::Arc;
 use std::time::Duration;
 use store::{HotColdDB, ItemStore};
+use task_executor::ShutdownReason;
 use types::{
     BeaconBlock, BeaconState, ChainSpec, EthSpec, Graffiti, Hash256, PublicKeyBytes, Signature,
     SignedBeaconBlock, Slot,
@@ -75,7 +76,7 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     eth1_chain: Option<Eth1Chain<T::Eth1Chain, T::EthSpec>>,
     event_handler: Option<ServerSentEventHandler<T::EthSpec>>,
     slot_clock: Option<T::SlotClock>,
-    shutdown_sender: Option<Sender<&'static str>>,
+    shutdown_sender: Option<Sender<ShutdownReason>>,
     head_tracker: Option<HeadTracker>,
     validator_pubkey_cache: Option<ValidatorPubkeyCache<T>>,
     spec: ChainSpec,
@@ -349,7 +350,7 @@ where
     }
 
     /// Sets a `Sender` to allow the beacon chain to send shutdown signals.
-    pub fn shutdown_sender(mut self, sender: Sender<&'static str>) -> Self {
+    pub fn shutdown_sender(mut self, sender: Sender<ShutdownReason>) -> Self {
         self.shutdown_sender = Some(sender);
         self
     }