feat: 拆分 stress benchmark 到细粒度单热点测试并修复 bench 采样逻辑

tests/stress/array_join.js

@@ -0,0 +1,12 @@
+var ITERATIONS = 50000;
+
+var sum = 0;
+for (var i = 0; i < ITERATIONS; i++) {
+  var arr = [];
+  for (var j = 0; j < 10; j++) {
+    arr.push(j);
+  }
+  var joined = arr.join(",");
+  sum += joined.length;
+}
+sum