benchmark,test: remove output from readable-async-iterator benchmark

Extra output makes test-benchmark-streams fail. Change console.log() to
console.assert().

Fixes: #34409

PR-URL: #34411
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>

Author: Trott

benchmark/streams/readable-async-iterator.js

@@ -34,9 +34,8 @@ async function main({ n, sync }) {
     }
   }
 
-  // Side effect to ensure V8 does not optimize away the
-  // loop as a noop.
-  console.log(x);
+  // Use x to ensure V8 does not optimize away the loop as a noop.
+  console.assert(x);
 
   bench.end(n);
 }