Tests: match concurrency with CPU count

Author: lovell

test/bench/perf.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const os = require('os');
 const fs = require('fs');
 
 const async = require('async');
@@ -25,6 +26,9 @@ const height = 588;
 // Disable libvips cache to ensure tests are as fair as they can be
 sharp.cache(false);
 
+// Spawn one thread per CPU
+sharp.concurrency(os.cpus().length);
+
 async.series({
   jpeg: function (callback) {
     const inputJpgBuffer = fs.readFileSync(fixtures.inputJpg);