benchmark: fix http/headers.js with test-double

The http/headers.js benchmark fails if wrk or autocannon are not
installed. This is because the benchmark exceeds the default permissible
length for HTTP headers. Reduce the largest benchmarks to fit within the
8192 default as that is what we should be optimizing for.

Fixes: #31022

PR-URL: #36794
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

Author: Trott

benchmark/http/headers.js

@@ -4,7 +4,7 @@ const common = require('../common.js');
 const http = require('http');
 
 const bench = common.createBenchmark(main, {
-  n: [10, 1000],
+  n: [10, 600],
   len: [1, 100],
   duration: 5
 });