Temporal: added a test reproducing tc39/proposal-temporal#2508

Before the issue is fixed, rounding a duration such as 'PT2400h' with
months as largest unit fails to balance it properly and gives a result
with days being the largest unit. The test ensures that such rounding
happens properly.

test/built-ins/Temporal/Duration/prototype/round/largestunit-correct-rebalancing.js

@@ -0,0 +1,24 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.duration.prototype.round
+description: Balancing from hours or smaller to weeks or bigger happens correctly.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const day_duration = 100;
+
+const tests = [ ["days", { days: day_duration }],
+  ["hours", { hours: day_duration * 24 }],
+  ["minutes", { minutes: day_duration * 24 * 60 }],
+  ["seconds", { seconds: day_duration * 24 * 60 * 60 }],
+  ["milliseconds", { milliseconds: day_duration * 24 * 60 * 60 * 1000 }],
+  ["microseconds", { microseconds: day_duration * 24 * 60 * 60 * 1000 * 1000 }],
+  ["nanoseconds", { nanoseconds: day_duration * 24 * 60 * 60 * 1000 * 1000 * 1000 }]];
+
+for ([unit, duration_desc] of tests)
+  TemporalHelpers.assertDuration(Temporal.Duration.from(duration_desc).round({ relativeTo: '2023-02-21', largestUnit: 'month' }),
+    0, 3, 0, 11, 0, 0, 0, 0, 0, 0, `rounding from ${unit}`);
+