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/errors.rs

@@ -21,6 +21,7 @@ use state_processing::{
     BlockProcessingError, SlotProcessingError,
 };
 use std::time::Duration;
+use task_executor::ShutdownReason;
 use types::*;
 
 macro_rules! easy_from_to {
@@ -96,7 +97,7 @@ pub enum BeaconChainError {
         head_block_epoch: Epoch,
     },
     WeakSubjectivtyVerificationFailure,
-    WeakSubjectivtyShutdownError(TrySendError<&'static str>),
+    WeakSubjectivtyShutdownError(TrySendError<ShutdownReason>),
     AttestingPriorToHead {
         head_slot: Slot,
         request_slot: Slot,