## 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.
