Temporal: Add tests for mixed signs in DifferenceTemporalPlainDateTime

With test cases kindly provided by Anba, this adds test coverage for the
abrupt completion in the last step of DifferenceTemporalPlainDateTime,
where the resulting Duration components have mixed signs.

See: https://github.com/tc39/proposal-temporal/issues/2783

test/built-ins/Temporal/PlainDateTime/prototype/since/result-mixed-sign.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.since
+description: >
+  RangeError when inconsistent custom calendar method causes mixed signs of
+  Duration components
+features: [Temporal]
+---*/
+
+// Test case provided by André Bargull
+
+const cal = new (class extends Temporal.Calendar {
+  dateAdd(date, duration, options) {
+    return super.dateAdd(date, duration.negated(), options);
+  }
+})("iso8601");
+
+const one = new Temporal.PlainDateTime(2000, 1, 1, 0, 0, 0, 0, 0, 0, cal);
+const two = new Temporal.PlainDateTime(2020, 5, 10, 12, 12, 0, 0, 0, 0, cal);
+
+assert.throws(RangeError, () => two.since(one, {
+  largestUnit: "years",
+  smallestUnit: "hours"
+}));

test/built-ins/Temporal/PlainDateTime/prototype/until/result-mixed-sign.js

@@ -0,0 +1,26 @@
+// Copyright (C) 2024 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindatetime.prototype.until
+description: >
+  RangeError when inconsistent custom calendar method causes mixed signs of
+  Duration components
+features: [Temporal]
+---*/
+
+// Test case provided by André Bargull
+
+const cal = new (class extends Temporal.Calendar {
+  dateAdd(date, duration, options) {
+    return super.dateAdd(date, duration.negated(), options);
+  }
+})("iso8601");
+
+const one = new Temporal.PlainDateTime(2000, 1, 1, 0, 0, 0, 0, 0, 0, cal);
+const two = new Temporal.PlainDateTime(2020, 5, 10, 12, 12, 0, 0, 0, 0, cal);
+
+assert.throws(RangeError, () => one.until(two, {
+  largestUnit: "years",
+  smallestUnit: "hours"
+}));