Fix class name check for WeakMap and WeakSet prototype (#3358)

Fixes #3357.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

tests/jerry/es2015/weakmap.js

@@ -135,6 +135,8 @@ try {
   assert (e instanceof TypeError);
 }
 
+assert (WeakMap.prototype.toString() === "[object WeakMap]");
+
 WeakMap.prototype.set = function () { throw "abrupt set" };
 
 try {

tests/jerry/es2015/weakset.js

@@ -119,6 +119,8 @@ try {
   assert (e instanceof TypeError);
 }
 
+assert (WeakSet.prototype.toString() === "[object WeakSet]");
+
 WeakSet.prototype.add = function () { throw "abrupt add" };
 
 try {