Test positive and negative offset time zones in FormatTimeZoneOffset

Tests for the normative change made to Temporal in
https://github.com/tc39/proposal-temporal/pull/1833

test/built-ins/Temporal/Instant/prototype/toString/timezone-offset.js

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+// 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
+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("UTC", "1970-01-01T00:00:00+00:00", "offset of UTC is +00:00");
+test("+01:00", "1970-01-01T01:00:00+01:00", "positive offset");
+test("-05:00", "1969-12-31T19:00:00-05:00", "negative offset");

test/built-ins/Temporal/TimeZone/prototype/getOffsetStringFor/basic.js

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+// 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.timezone.prototype.getoffsetstringfor
+description: Basic tests for Temporal.TimeZone.prototype.getOffsetStringFor
+features: [BigInt, Temporal]
+---*/
+
+const instant = new Temporal.Instant(0n);
+
+function test(timeZoneIdentifier, expectedOffsetString, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  assert.sameValue(timeZone.getOffsetStringFor(instant), expectedOffsetString, description);
+}
+
+test("UTC", "+00:00", "offset of UTC is +00:00");
+test("+01:00", "+01:00", "positive offset");
+test("-05:00", "-05:00", "negative offset");

test/built-ins/Temporal/ZonedDateTime/prototype/getISOFields/offset.js

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+// 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.zoneddatetime.prototype.getisofields
+description: The offset property of returned object
+features: [BigInt, Temporal]
+---*/
+
+function test(timeZoneIdentifier, expectedOffsetString, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  const datetime = new Temporal.ZonedDateTime(0n, timeZone);
+  const fields = datetime.getISOFields();
+  assert.sameValue(fields.offset, expectedOffsetString, description);
+}
+
+test("UTC", "+00:00", "offset of UTC is +00:00");
+test("+01:00", "+01:00", "positive offset");
+test("-05:00", "-05:00", "negative offset");

test/built-ins/Temporal/ZonedDateTime/prototype/offset/basic.js

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+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-get-temporal.zoneddatetime.prototype.offset
+description: Basic tests for Temporal.ZonedDateTime.prototype.offset
+features: [BigInt, Temporal]
+---*/
+
+function test(timeZoneIdentifier, expectedOffsetString, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  const datetime = new Temporal.ZonedDateTime(0n, timeZone);
+  assert.sameValue(datetime.offset, expectedOffsetString, description);
+}
+
+test("UTC", "+00:00", "offset of UTC is +00:00");
+test("+01:00", "+01:00", "positive offset");
+test("-05:00", "-05:00", "negative offset");

test/built-ins/Temporal/ZonedDateTime/prototype/toJSON/offset.js

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+// 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.zoneddatetime.prototype.tojson
+description: The time zone offset part of the string serialization
+features: [BigInt, Temporal]
+---*/
+
+function test(timeZoneIdentifier, expected, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  const datetime = new Temporal.ZonedDateTime(0n, timeZone);
+  assert.sameValue(datetime.toJSON(), expected, description);
+}
+
+test("UTC", "1970-01-01T00:00:00+00:00[UTC]", "offset of UTC is +00:00");
+test("+01:00", "1970-01-01T01:00:00+01:00[+01:00]", "positive offset");
+test("-05:00", "1969-12-31T19:00:00-05:00[-05:00]", "negative offset");

test/built-ins/Temporal/ZonedDateTime/prototype/toString/offset.js

@@ -0,0 +1,18 @@
+// 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.zoneddatetime.prototype.tostring
+description: The time zone offset part of the string serialization
+features: [BigInt, Temporal]
+---*/
+
+function test(timeZoneIdentifier, expected, description) {
+  const timeZone = new Temporal.TimeZone(timeZoneIdentifier);
+  const datetime = new Temporal.ZonedDateTime(0n, timeZone);
+  assert.sameValue(datetime.toString(), expected, description);
+}
+
+test("UTC", "1970-01-01T00:00:00+00:00[UTC]", "offset of UTC is +00:00");
+test("+01:00", "1970-01-01T01:00:00+01:00[+01:00]", "positive offset");
+test("-05:00", "1969-12-31T19:00:00-05:00[-05:00]", "negative offset");