Fix bench errors in rust-analyzer

bin/benches/comparison_benches.rs

@@ -1,3 +1,4 @@
+#![cfg(nightly)]
 #![feature(test)]
 
 extern crate test;

crates/client/benches/lower_name_benches.rs

@@ -5,6 +5,7 @@
 // http://opensource.org/licenses/MIT>, at your option. This file may not be
 // copied, modified, or distributed except according to those terms.
 
+#![cfg(nightly)]
 #![feature(test)]
 
 extern crate test;

crates/proto/benches/lib.rs

@@ -1,3 +1,4 @@
+#![cfg(nightly)]
 #![feature(test)]
 
 extern crate test;

crates/proto/benches/name_benches.rs

@@ -1,3 +1,4 @@
+#![cfg(nightly)]
 #![feature(test)]
 
 extern crate test;