Temporal: Extend tests for PlainDate#toPlainYearMonth.

test/built-ins/Temporal/PlainDate/prototype/toPlainYearMonth/basic.js

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+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.toplainyearmonth
+description: Basic toPlainYearMonth tests.
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = new Temporal.Calendar("iso8601");
+const pd = new Temporal.PlainDate(1970, 12, 24, calendar);
+const pym = pd.toPlainYearMonth();
+TemporalHelpers.assertPlainYearMonth(pym, 1970, 12, "M12");
+assert.sameValue(pym.calendar, calendar);

test/built-ins/Temporal/PlainDate/prototype/toPlainYearMonth/calendar-invalid-return.js

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+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.toplainyearmonth
+description: Throw when the returned value is not a PlainYearMonth.
+features: [Temporal]
+---*/
+
+class CustomCalendar extends Temporal.Calendar {
+  constructor(value) {
+    super("iso8601");
+    this.value = value;
+  }
+  yearMonthFromFields() {
+    return this.value;
+  }
+}
+
+const tests = [
+  [undefined],
+  [null, "null"],
+  [true],
+  ["2000-05"],
+  [Symbol()],
+  [200005],
+  [200005n],
+  [{}, "plain object"],
+  [() => {}, "lambda"],
+  [Temporal.PlainYearMonth, "Temporal.PlainYearMonth"],
+  [Temporal.PlainYearMonth.prototype, "Temporal.PlainYearMonth.prototype"],
+];
+for (const [test, description = typeof test] of tests) {
+  const plainDate = new Temporal.PlainDate(2000, 5, 2, new CustomCalendar(test));
+  assert.throws(TypeError, () => plainDate.toPlainYearMonth(), `Expected error with ${description}`);
+}