tests/cfg: allow unused extern crates
tests/cfg: allow unused extern crates

nipunn1313/cargo,achanda/cargo,vojtechkral/cargo,tsurai/cargo,tsurai/cargo,tshepang/cargo,tsurai/cargo,achanda/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,vojtechkral/cargo,achanda/cargo,rustyhorde/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,achanda/cargo,vojtechkral/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,vojtechkral/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo