Mark some test-only code as such to avoid dead code warnings.
Mark some test-only code as such to avoid dead code warnings.

chris-morgan/rust-http,servo/rust-http,Shraddha512/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,servo/rust-http,Shraddha512/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,Shraddha512/rust-http,chris-morgan/rust-http,servo/rust-http,servo/rust-http