fix: 修复 coercion_concat/coercion_tostring/typeof_ops 压测字符串无限增长导致内存耗尽

tests/stress/coercion_concat.js

@@ -2,6 +2,7 @@ var ITERATIONS = 100000;
 
 var sum = 0;
 for (var i = 0; i < ITERATIONS; i++) {
-  sum += "42" + i;
+  var s = "42" + i;
+  sum += s.length;
 }
 sum

tests/stress/coercion_tostring.js

@@ -2,6 +2,7 @@ var ITERATIONS = 100000;
 
 var sum = 0;
 for (var i = 0; i < ITERATIONS; i++) {
-  sum += String(42) + String(3.14) + String(0) + String(true) + String(null);
+  var s = String(42) + String(3.14) + String(0) + String(true) + String(null);
+  sum += s.length;
 }
 sum

tests/stress/typeof_ops.js

@@ -2,9 +2,9 @@ var ITERATIONS = 100000;
 
 var sum = 0;
 for (var i = 0; i < ITERATIONS; i++) {
-  sum += typeof i;
-  sum += typeof "str";
-  sum += typeof null;
-  sum += typeof {}.a;
+  sum += (typeof i).length;
+  sum += (typeof "str").length;
+  sum += (typeof null).length;
+  sum += (typeof {}).length;
 }
 sum