Feature gate test CLI flags (#8231)

Closes #6980


  I think these flags may be useful in future peerdas / das testing, and would be useful to keep. Hence I've gated them behind a `testing` feature flag.


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

lighthouse/Cargo.toml

@@ -81,6 +81,7 @@ malloc_utils = { workspace = true, features = ["jemalloc"] }
 malloc_utils = { workspace = true, features = [] }
 
 [dev-dependencies]
+beacon_node = { workspace = true, features = ["testing"] }
 beacon_node_fallback = { workspace = true }
 beacon_processor = { workspace = true }
 eth2 = { workspace = true }