CLI tests for logging flags

[michaelsproul]

Description

Two recent PRs have added new logging flags which can't be tested by our current CLI testing framework due to the LoggerConfig not being part of the client::Config that gets persisted to disk.

The flags that would be good to test are:

• [Merged by Bors] - Add flag 'log-color' preserving color of log redirected to file.  #3538
• [Merged by Bors] - Add disable-log-timestamp flag (#3101) #3586

Steps to resolve

Add the LoggerConfig to client::Config here:

lighthouse/beacon_node/client/src/config.rs

Line 44 in aa022f4

pub struct Config {

Do some plumbing so that the logger config is copied into the client config on startup (probably in lighthouse/src/main.rs).

Add two tests for each of the new flags: one checking the default value of the config without the flag and another checking that the config changes when the flag is provided (in lighthouse/tests/beacon_node.rs).