Added new tests to reflect the change that daysInMonth is the count of days in a month

This change affects how PlainYearMonth's add and subtract behave. They
now ignore daysInMonth reimplementations in the calendar, and they call
dateadd() a different number of times.

See https://github.com/tc39/proposal-temporal/issues/1315

test/built-ins/Temporal/PlainYearMonth/prototype/add/calendar-dateadd-called-with-plaindate-instance.js

@@ -0,0 +1,18 @@
+// 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.plainyearmonth.prototype.add
+description: Duration addition to PlainYearMonth calls Calendar.dateAdd the right number of times
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarDateAddPlainDateInstance();
+const instance = new Temporal.PlainYearMonth(1983, 3, calendar);
+TemporalHelpers.assertPlainYearMonth(instance.add({days: 31}), 1983, 4, 'M04', "Adding 31 days to march in is8601 calendar")
+assert.sameValue(calendar.dateAddCallCount, 1, "dateAdd called once with positive add");
+
+calendar.dateAddCallCount = 0;
+TemporalHelpers.assertPlainYearMonth(instance.add({days: -31}), 1983, 2, 'M02', "Adding -31 days to march in is8601 calendar")
+assert.sameValue(calendar.dateAddCallCount, 3, "dateAdd called 3 times with negative add");

test/built-ins/Temporal/PlainYearMonth/prototype/add/custom-daysInMonth-irrelevant.js

@@ -0,0 +1,23 @@
+// 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.plainyearmonth.prototype.add
+description: Addition of a negative duration to a PlainYearMonth is not influenced by the implementation of daysInMonth()
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+class CustomCalendar extends Temporal.Calendar {
+  constructor() {
+    super("iso8601");
+  }
+  daysInMonth(ym, ...args) {
+    return 15;
+  }
+}
+
+const customCalendar = new CustomCalendar();
+const instance = new Temporal.PlainYearMonth(2023, 3, customCalendar);
+
+TemporalHelpers.assertPlainYearMonth(instance.add({days: -30}), 2023, 3, 'M03', "Adding -30 days from calendar reimplementing daysinMonth()")

test/built-ins/Temporal/PlainYearMonth/prototype/subtract/calendar-dateadd-called-with-plaindate-instance.js

@@ -0,0 +1,18 @@
+// 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.plainyearmonth.prototype.subtract
+description: Duration subtraction from PlainYearMonth calls Calendar.dateAdd the right number of times
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarDateAddPlainDateInstance();
+const instance = new Temporal.PlainYearMonth(1983, 3, calendar);
+TemporalHelpers.assertPlainYearMonth(instance.subtract({days: 31}), 1983, 2, 'M02', "Removing 31 days to march in is8601 calendar")
+assert.sameValue(calendar.dateAddCallCount, 3, "dateAdd called 3 times with positive subtract");
+
+calendar.dateAddCallCount = 0;
+TemporalHelpers.assertPlainYearMonth(instance.subtract({days: -31}), 1983, 4, 'M04', "Removing -31 days to march in is8601 calendar")
+assert.sameValue(calendar.dateAddCallCount, 1, "dateAdd called once with negative subtract");

test/built-ins/Temporal/PlainYearMonth/prototype/subtract/custom-daysInMonth-irrelevant.js

@@ -0,0 +1,23 @@
+// 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.plainyearmonth.prototype.subtract
+description: Subtraction of positive duration to a PlainYearMonth is not influenced by the implementation of daysInMonth()
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+class CustomCalendar extends Temporal.Calendar {
+  constructor() {
+    super("iso8601");
+  }
+  daysInMonth(ym, ...args) {
+    return 15;
+  }
+}
+
+const customCalendar = new CustomCalendar();
+const instance = new Temporal.PlainYearMonth(2023, 3, customCalendar);
+
+TemporalHelpers.assertPlainYearMonth(instance.subtract({days: 30}), 2023, 3, 'M03', "Subtracting 30 days from calendar reimplementing daysinMonth()")