CLI tests for logging flags (#3609)

## Issue Addressed
Adding CLI tests for logging flags: log-color and disable-log-timestamp
Which issue # does this PR address?
#3588 
## Proposed Changes
Add CLI tests for logging flags as described in #3588 
Please list or describe the changes introduced by this PR.
Added logger_config to client::Config as suggested. Implemented Default for LoggerConfig based on what was being done elsewhere in the repo. Created 2 tests for each flag addressed.
## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

beacon_node/client/src/config.rs

@@ -1,11 +1,11 @@
 use directory::DEFAULT_ROOT_DIR;
+use environment::LoggerConfig;
 use network::NetworkConfig;
 use sensitive_url::SensitiveUrl;
 use serde_derive::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
 use types::{Graffiti, PublicKeyBytes};
-
 /// Default directory name for the freezer database under the top-level data dir.
 const DEFAULT_FREEZER_DB_DIR: &str = "freezer_db";
 
@@ -72,6 +72,7 @@ pub struct Config {
     pub http_metrics: http_metrics::Config,
     pub monitoring_api: Option<monitoring_api::Config>,
     pub slasher: Option<slasher::Config>,
+    pub logger_config: LoggerConfig,
 }
 
 impl Default for Config {
@@ -96,6 +97,7 @@ impl Default for Config {
             slasher: None,
             validator_monitor_auto: false,
             validator_monitor_pubkeys: vec![],
+            logger_config: LoggerConfig::default(),
         }
     }
 }