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>

beacon_node/Cargo.toml

@@ -15,6 +15,7 @@ path = "src/lib.rs"
 write_ssz_files = [
     "beacon_chain/write_ssz_files",
 ] # Writes debugging .ssz files to /tmp during block processing.
+testing = [] # Enables testing-only CLI flags
 
 [dependencies]
 account_utils = { workspace = true }