Add regression tests for boot_node (#2749)

## Issue Addressed
Resolves #2602

## Proposed Changes

*Note: For a review it might help to look at the individual commits.*

### `boot_node`
Add support for the flags `dump-config` and `immediate-shutdown`. For `immediate-shutdown` the actual behavior could be described as `dump-config-and-exit`.

Both flags are handled in `boot_node::main`, which appears to be the simplest approach.

### `boot_node` regression tests
Added in `lighthouse/tests/boot_node.rs`.

### `CommandLineTestExec`
Factors out boilerplate related to CLI tests. It's used in the regression tests for `boot_node`, `beacon_node` and `validator_client`.

## Open TODO
Add tests for `boot_node` flags `enable-enr-auto-update` and `disable-packet-filter`. They end up in [`Discv5Config`](9ed2cba6bc/boot_node/src/config.rs (L29)), which doesn't support serde (de)serialization.

I haven't found a workaround - guidance would be appreciated.

Cargo.lock

@@ -479,11 +479,15 @@ version = "2.0.1"
 dependencies = [
  "beacon_node",
  "clap",
+ "clap_utils",
  "eth2_ssz",
  "hex",
  "lighthouse_network",
  "log",
  "logging",
+ "serde",
+ "serde_derive",
+ "serde_json",
  "slog",
  "slog-async",
  "slog-scope",
@@ -3193,6 +3197,7 @@ dependencies = [
  "lighthouse_network",
  "lighthouse_version",
  "malloc_utils",
+ "serde",
  "serde_json",
  "slashing_protection",
  "slog",