Correct test for `for-in` IterationStatement

This test was intended to assert the semantics of the `for-in`
statement, but it was mistakenly authored to use a `for-of` statement.
Update the statement under test, and improve the testing methodology to
correctly assert the creation of distinct bindings.

test/language/statements/for-in/head-let-fresh-binding-per-iteration.js

 // Copyright (C) 2011 the V8 project authors. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.
 /*---
-es6id: 13.6.4.13
+esid: sec-runtime-semantics-forin-div-ofbodyevaluation-lhs-stmt-iterator-lhskind-labelset
+es6id: 13.7.5.13
 description: >
     let ForDeclaration: creates a fresh binding per iteration
 ---*/
 
-let s = '';
-for (let x of [1, 2, 3]) {
-  s += x;
-}
-assert.sameValue(s, '123', "The value of `s` is `'123'`");
+var fns = {};
+var obj = Object.create(null);
+obj.a = 1;
+obj.b = 1;
+obj.c = 1;
 
+for (let x in obj) {
+  // Store function objects as properties of an object so that their return
+  // value may be verified regardless of the for-in statement's enumeration
+  // order.
+  fns[x] = function() { return x; };
+}
 
+assert.sameValue(typeof fns.a, 'function', 'property definition: "a"');
+assert.sameValue(fns.a(), 'a');
+assert.sameValue(typeof fns.b, 'function', 'property definition: "b"');
+assert.sameValue(fns.b(), 'b');
+assert.sameValue(typeof fns.c, 'function', 'property definition: "c"');
+assert.sameValue(fns.c(), 'c');