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.

lighthouse/src/main.rs

@@ -204,7 +204,7 @@ fn main() {
                 .expect("Debug-level must be present")
                 .into();
 
-            boot_node::run(bootnode_matches, eth_spec_id, debug_info);
+            boot_node::run(&matches, bootnode_matches, eth_spec_id, debug_info);
 
             return Ok(());
         }