hwtest.js: Fix median computation

By default Array.prototype.sort() sorts all values by converting
them to strings, even numbers!

js/perf/hwtest.js

@@ -270,7 +270,7 @@ function script_redrawTestDone(time) {
 function script_collectTimings(time) {
     for (let timing in redrawTimes) {
         let times = redrawTimes[timing];
-        times.sort();
+        times.sort(function(a, b) { return a - b });
 
         let len = times.length;
         let median;