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.

boot_node/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 [dependencies]
 beacon_node = { path = "../beacon_node" }
 clap = "2.33.3"
+clap_utils = { path = "../common/clap_utils" }
 lighthouse_network = { path = "../beacon_node/lighthouse_network" }
 types = { path = "../consensus/types" }
 eth2_ssz = "0.4.0"
@@ -19,3 +20,6 @@ slog-async = "2.5.0"
 slog-scope = "4.3.0"
 slog-stdlog = "4.0.0"
 hex = "0.4.2"
+serde = "1.0.116"
+serde_derive = "1.0.116"
+serde_json = "1.0.66"