Set subscribers defaults instead of using once

Use `tracing::subscriber::set_default` for setting the subscriber on a
thread/test.

Using `std::sync::Once` would cause any subsequent tests that try to use
a different level won't see their preferred level applied, and it would
not be obvious why debug logs aren't showing.

Author: danielrachi1

beacon_node/lighthouse_network/tests/common.rs

@@ -13,9 +13,6 @@ use types::{
     ChainSpec, EnrForkId, Epoch, EthSpec, FixedBytesExtended, ForkContext, ForkName, Hash256,
     MinimalEthSpec,
 };
-use std::sync::Once;
-
-static INIT: Once = Once::new();
 
 type E = MinimalEthSpec;
 
@@ -77,11 +74,11 @@ impl std::ops::DerefMut for Libp2pInstance {
 #[allow(unused)]
 pub fn build_tracing_subscriber(level: &str, enabled: bool) {
     if enabled {
-        INIT.call_once(|| {
+        tracing::subscriber::set_default(
             tracing_subscriber::fmt()
                 .with_env_filter(EnvFilter::try_new(level).unwrap())
-                .init();
-        });
+                .finish()
+        );
     }
 }