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_split.js

@@ -0,0 +1,9 @@
+var ITERATIONS = 20000;
+
+var s = "a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z";
+var sum = 0;
+for (var i = 0; i < ITERATIONS; i++) {
+  var parts = s.split(",");
+  sum += parts.length;
+}
+sum