Temporal: Add coverage for getTimeZoneTransition edge cases (#4466)

getTimeZoneTransition return values should always correspond with UTC
offset transitions, not with time zone metadata changes such as making
DST permanent or updating abbreviation strings.

See: https://github.com/tc39/proposal-temporal/issues/3105

test/intl402/Temporal/ZonedDateTime/prototype/getTimeZoneTransition/rule-change-without-offset-transition.js

@@ -0,0 +1,70 @@
+// Copyright (C) 2022 André Bargull. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.zoneddatetime.prototype.gettimezonetransition
+description: >
+  Rule changes in the TZDB that do not have offset transtions should not be
+  returned by getTimeZoneTransition.
+features: [Temporal]
+---*/
+
+// Europe/London changed from DST to permanent British Standard Time on
+// 1968-10-27, but the actual UTC offset did not change at that time.
+// getTimeZoneTransition should not return an instant on 1968-10-27.
+
+const londonPrev = new Temporal.ZonedDateTime(0n, "Europe/London")
+  .getTimeZoneTransition("previous");
+assert.notSameValue(
+  londonPrev.offsetNanoseconds,
+  londonPrev.subtract({ nanoseconds: 1 }).offsetNanoseconds,
+  "should be a UTC offset transition"
+);
+assert.sameValue(
+  londonPrev.epochNanoseconds,
+  -59004000000000000n,
+  "epoch nanoseconds for 1968-02-18T03:00:00+01:00"
+);
+
+const londonNext = new Temporal.ZonedDateTime(-39488400000000000n /* 1968-10-01T00:00:00+01:00 */, "Europe/London")
+  .getTimeZoneTransition("next");
+assert.notSameValue(
+  londonNext.offsetNanoseconds,
+  londonNext.subtract({ nanoseconds: 1 }).offsetNanoseconds,
+  "should be a UTC offset transition"
+);
+assert.sameValue(
+  londonNext.epochNanoseconds,
+  57722400000000000n,
+  "epoch nanoseconds for 1971-10-31T02:00:00+00:00"
+);
+
+// Similarly, America/Anchorage changed from DST to permanent standard time on
+// 1967-04-01. The UTC offset did not change, but the abbreviation did (AST to
+// AHST). Still, getTimeZoneTransition should not return an instant on 1967-04-01
+
+const anchoragePrev = new Temporal.ZonedDateTime(-84290400000000000n /* 1967-05-01T00:00:00-10:00 */, "America/Anchorage")
+  .getTimeZoneTransition("previous");
+assert.notSameValue(
+  anchoragePrev.offsetNanoseconds,
+  anchoragePrev.subtract({ nanoseconds: 1 }).offsetNanoseconds,
+  "should be a UTC offset transition"
+);
+assert.sameValue(
+  anchoragePrev.epochNanoseconds,
+  -765378000000000000n,
+  "epoch nanoseconds for 1945-09-30T01:00:00-10:00"
+);
+
+const anchorageNext = new Temporal.ZonedDateTime(-94658400000000000n /* 1967-01-01T00:00:00-10:00 */, "America/Anchorage")
+  .getTimeZoneTransition("next");
+assert.notSameValue(
+  anchorageNext.offsetNanoseconds,
+  anchorageNext.subtract({ nanoseconds: 1 }).offsetNanoseconds,
+  "should be a UTC offset transition"
+);
+assert.sameValue(
+  anchorageNext.epochNanoseconds,
+  -21470400000000000n,
+  "epoch nanoseconds for 1969-04-27T03:00:00-09:00"
+);