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>

Cargo.lock

@@ -3719,6 +3719,7 @@ dependencies = [
  "directory",
  "env_logger 0.8.3",
  "environment",
+ "eth2_libp2p",
  "eth2_network_config",
  "futures 0.3.14",
  "lazy_static",
@@ -3726,11 +3727,13 @@ dependencies = [
  "lighthouse_version",
  "logging",
  "remote_signer",
+ "serde_json",
  "slashing_protection",
  "slog",
  "slog-async",
  "slog-term",
  "sloggers",
+ "task_executor",
  "tempfile",
  "tokio 1.5.0",
  "types",