Repeat Instant.p.toString() test with timeZone parameter for IANA time zones

This adds to the intl402/ tree a copy of
built-ins/Temporal/Instant/prototype/toString/timezone-offset.js but which
uses IANA time zones.

test/intl402/Temporal/Instant/prototype/toString/timezone-offset.js

@@ -0,0 +1,19 @@
+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.instant.prototype.tostring
+description: The time zone offset part of the string serialization (Intl time zones)
+features: [BigInt, Temporal]
+---*/
+
+const instant = new Temporal.Instant(0n);
+
+function test(timeZoneIdentifier, expected, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  assert.sameValue(instant.toString({ timeZone }), expected, description);
+}
+
+test("Europe/Berlin", "1970-01-01T01:00:00+01:00", "positive offset");
+test("America/New_York", "1969-12-31T19:00:00-05:00", "negative offset");
+test("Africa/Monrovia", "1969-12-31T23:15:30-00:45", "sub-minute offset");