Check a variety of offset Etc/GMT timezones (#3403)

Tests for normative change https://github.com/tc39/proposal-temporal/pull/2050

test/intl402/Temporal/TimeZone/etc-timezone.js

@@ -0,0 +1,79 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.timezone
+description: Some Etc/GMT{+/-}{0}N timezones are valid, but not all
+features: [Temporal]
+---*/
+
+// "Etc/GMT-0" through "Etc/GMT-14" are OK
+
+assert.sameValue(
+  (new Temporal.TimeZone("Etc/GMT-0")).toString(),
+  "UTC", // if the offset is -0, we say "UTC" rather than "GMT"
+  "Etc/GMT-0 is a valid timezone"
+);
+
+[1,2,3,4,5,6,7,8,9,10,11,12,13,14].forEach((n) => {
+  let tz = "Etc/GMT-" + n;
+  let instance = new Temporal.TimeZone(tz);
+  assert.sameValue(
+    instance.toString(),
+    tz,
+    tz + " is a valid timezone"
+  );
+});
+
+let gmtMinus24TZ = "Etc/GMT-24";
+assert.throws(
+  RangeError,
+  () => { new Temporal.TimeZone(gmtMinus24TZ); },
+  gmtMinus24TZ + " is an invalid timezone"
+);
+
+// "Etc/GMT-0N" is not OK (1 ≤ N ≤ 9)
+[1,2,3,4,5,6,7,8,9].forEach((n) => {
+  let tz = "Etc/GMT-0" + n;
+  assert.throws(
+    RangeError,
+    () => { new Temporal.TimeZone(tz); },
+    tz + " is an invalid timezone"
+  );
+});
+
+// "Etc/GMT+0N" is not OK (0 ≤ N ≤ 9)
+[0,1,2,3,4,5,6,7,8,9].forEach((n) => {
+  let tz = "Etc/GMT+0" + n;
+  assert.throws(
+    RangeError,
+    () => { new Temporal.TimeZone(tz); },
+    tz + " is an invalid timezone"
+    );
+});
+
+// Etc/GMT+0" through "Etc/GMT+12" are OK
+
+// zero is handled in its own way (say "UTC" rather than "GMT"):
+assert.sameValue(
+  (new Temporal.TimeZone("Etc/GMT+0")).toString(),
+  "UTC", // if the offset is +0, we say "UTC" rather than "GMT"
+  "Etc/GMT+0 is a valid timezone"
+);
+
+[1,2,3,4,5,6,7,8,9,10,11,12].forEach((n) => {
+  let tz = "Etc/GMT+" + n;
+  let instance = new Temporal.TimeZone(tz);
+  assert.sameValue(
+    instance.toString(),
+    tz,
+    tz + " is a valid timezone"
+  );
+});
+
+let gmtPlus24TZ = "Etc/GMT+24";
+assert.throws(
+  RangeError,
+  () => { new Temporal.TimeZone(gmtPlus24TZ); },
+  gmtPlus24TZ + " is an invalid timezone"
+);