Add test to validate fix to proposal-temporal#2383

The Temporal polyfill had an infinite loop for non-ISO calendars
when Calendar.p.dateUntil was called to calculate the duration
between two identical dates. This test validates that PT0S is returned
in that case.

test/intl402/Temporal/Calendar/prototype/dateUntil/zero-length-duration-result.js

@@ -0,0 +1,16 @@
+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: The duration from a date to itself is a zero duration (PT0S)
+esid: sec-temporal.calendar.prototype.dateuntil
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Calendar("gregory");
+const date = new Temporal.PlainDate(2001, 6, 3);
+
+['year', 'month', 'week', 'day'].forEach((largestUnit) => {
+  const result = instance.dateUntil(date, date, { largestUnit });
+  assert.sameValue(result.toString(), 'PT0S');
+});