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

tests/stress/loop_for_of_string.js

@@ -0,0 +1,13 @@
+var ITERATIONS = 1500;
+
+var s = "";
+for (var i = 0; i < 50; i++) {
+  s += "ab";
+}
+var sum = 0;
+for (var j = 0; j < ITERATIONS; j++) {
+  for (var k of s) {
+    sum += k.charCodeAt(0);
+  }
+}
+sum