Temporal: Add test coverage for edge case in DST balancing after roun…

…ding

This covers an edge case that we hit, where 24 hours would not balance up
to one day in a 25-hour day if only largestUnit was specified, but would
erroneously balance up if rounding was also performed by specifying
smallestUnit.

test/built-ins/Temporal/Duration/prototype/round/dst-balancing-result.js

@@ -22,3 +22,20 @@ const timeZone = TemporalHelpers.springForwardFallBackTimeZone();
     TemporalHelpers.assertDuration(result, 1, 0, 0, 0, 24, 0, 0, 0, 0, 0,
         "24 hours does not balance to 1 day in 25-hour day");
 }
+
+{
+  const duration = new Temporal.Duration(0, 0, 0, 0, /* hours = */ 24, 0, 0, 0, 0, /* ns = */ 5);
+  const relativeTo = new Temporal.ZonedDateTime(
+    972802800_000_000_000n /* = 2000-10-29T07Z */,
+    timeZone); /* = 2000-10-29T00-07 in local time */
+
+  const result = duration.round({
+    largestUnit: "days",
+    smallestUnit: "minutes",
+    roundingMode: "expand",
+    roundingIncrement: 30,
+    relativeTo
+  });
+  TemporalHelpers.assertDuration(result, 0, 0, 0, 0, 24, 30, 0, 0, 0, 0,
+    "24 hours does not balance after rounding to 1 day in 25-hour day");
+}