intl: increase coverage for NumberFormat.format

Increase test coverage for Intl.NumberFormat.prototype.format by adding
a check for testing if it calls ToNumber (7.1.3) on its argument.

test/intl402/NumberFormat/prototype/format/value-tonumber.js

@@ -0,0 +1,37 @@
+// Copyright (C) 2018 Ujjwal Sharma. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-number-format-functions
+description: >
+  Tests that Intl.NumberFormat.prototype.format converts its argument
+  (called value) to a number using ToNumber (7.1.3).
+info: |
+  11.1.4Number Format Functions
+
+  4. Let x be ? ToNumber(value).
+features: [Symbol]
+---*/
+
+const toNumberResults = [
+  [undefined, NaN],
+  [null, +0],
+  [true, 1],
+  [false, 0],
+  ['42', 42],
+  ['foo', NaN]
+];
+
+const nf = new Intl.NumberFormat();
+
+toNumberResults.forEach(pair => {
+  const value = pair[0];
+  const result = pair[1];
+  assert.sameValue(nf.format(value), nf.format(result));
+});
+
+assert.throws(
+  TypeError,
+  () => nf.format(Symbol()),
+  "ToNumber(arg) throws a TypeError when arg is of type 'Symbol'"
+);