Add test to validate fix to proposal-temporal#2537

The Temporal polyfill had a math bug in Calendar.p.dateUntil where
non-ISO calendars would unexpectedly throw when calculating
the number of years between two dates where the year were different,
the month codes were different, but the months were the same because
the earlier date's month was after a leap month.

This test validates the fix to this bug.

test/intl402/Temporal/Calendar/prototype/dateUntil/until-across-lunisolar-leap-months.js

@@ -0,0 +1,22 @@
+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: dateUntil works as expected after a leap month in a lunisolar calendar
+esid: sec-temporal.calendar.prototype.dateuntil
+features: [Temporal]
+---*/
+
+const instance = new Temporal.Calendar("chinese");
+
+// 2001 is a leap year in the Chinese calendar with a M04L leap month.
+// Therefore, month: 6 is M05 in 2001 but M06 in 2000 which is not a leap year.
+const one = Temporal.PlainDate.from({ year: 2000, month: 6, day: 1, calendar: 'chinese' });
+const two = Temporal.PlainDate.from({ year: 2001, month: 6, day: 1, calendar: 'chinese' });
+
+const expected = { years: 'P12M', months: 'P12M', weeks: 'P50W4D', days: 'P354D' };
+
+Object.entries(expected).forEach(([largestUnit, expectedResult]) => {
+  const actualResult = instance.dateUntil(one, two, { largestUnit });
+  assert.sameValue(actualResult.toString(), expectedResult);
+});