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

tests/stress/string_substring.js

@@ -0,0 +1,9 @@
+var ITERATIONS = 50000;
+
+var s = "The quick brown fox jumps over the lazy dog. 0123456789 abcdefghijklmnopqrstuvwxyz";
+var sum = 0;
+for (var i = 0; i < ITERATIONS; i++) {
+  var sub = s.substring(4, 15);
+  sum += sub.length;
+}
+sum