intl: increase coverage for the Intl.NumberFormat constructor

Increase coverge for the Intl.NumberFormat constructor by testing that
Get(O, P) and ToString(arg) are properly called.

test/intl402/NumberFormat/constructor-locales-get-tostring.js

+// Copyright (C) 2018 Ujjwal Sharma. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-initializenumberformat
+description: >
+  Tests that Get(O, P) and ToString(arg) are properly called within the
+  constructor for Intl.NumberFormat
+info: |
+  9.2.1 CanonicalizeLocaleList ( locales )
+
+  5. Let len be ? ToLength(? Get(O, "length")).
+
+  7.a. Let Pk be ToString(k).
+
+  7.c.i. Let kValue be ? Get(O, Pk).
+---*/
+
+const locales = {
+  length: 8,
+  1: 'en-US',
+  3: 'de-DE',
+  5: 'en-IN',
+  7: 'en-GB'
+};
+
+const actualLookups = [];
+const expectedLookups = Object.keys(locales);
+
+const handlers = {
+  get(obj, prop) {
+    actualLookups.push(prop);
+    return Reflect.get(...arguments);
+  }
+};
+
+const proxyLocales = new Proxy(locales, handlers);
+
+const nf = new Intl.NumberFormat(proxyLocales);
+
+actualLookups.forEach(lookup => assert(expectedLookups.indexOf(lookup) != -1));