intl: add assertion to test that the result for default parameter

Add an assertion to the test for default parameters in Intl.NumberFormat's
method formatToParts checking that both the values are equal to the correct
result.

test/intl402/NumberFormat/prototype/formatToParts/default-parameter.js

 // Copyright (C) 2017 Josh Wolfe. All rights reserved.
 // This code is governed by the BSD license found in the LICENSE file.
+
 /*---
 esid: sec-intl.numberformat.prototype.formattoparts
 description: Intl.NumberFormat.prototype.formatToParts called with no parameters
@@ -11,12 +12,19 @@ info: |
 
 var nf = new Intl.NumberFormat();
 
-// Example value: [{"type":"nan","value":"NaN"}]
-var implicit = nf.formatToParts();
-var explicit = nf.formatToParts(undefined);
+const implicit = nf.formatToParts();
+const explicit = nf.formatToParts(undefined);
+const result = [{ type: 'nan', value: 'NaN' }];
+
+assert(
+  partsEquals(implicit, explicit),
+  'formatToParts() should be equivalent to formatToParts(undefined)'
+);
 
-assert(partsEquals(implicit, explicit),
-  "formatToParts() should be equivalent to formatToParts(undefined)");
+assert(
+  partsEquals(implicit, result),
+  'Both implicit and explicit calls should have the correct result'
+);
 
 function partsEquals(parts1, parts2) {
   if (parts1.length !== parts2.length) return false;