Add tests for PlainDate#toString().

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

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+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.protoype.tostring
+description: basic tests
+features: [Temporal]
+---*/
+
+const date1 = new Temporal.PlainDate(1976, 11, 18);
+assert.sameValue(date1.toString(), "1976-11-18");
+
+const date2 = new Temporal.PlainDate(1914, 2, 23);
+assert.sameValue(date2.toString(), "1914-02-23");
+
+const date3 = new Temporal.PlainDate(1996, 2, 29);
+assert.sameValue(date3.toString(), "1996-02-29");

test/built-ins/Temporal/PlainDate/prototype/toString/calendarname-always.js

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+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.protoype.tostring
+description: always value for calendarName option
+features: [Temporal]
+---*/
+
+const calendar = {
+  toString() { return "custom"; }
+};
+const date1 = new Temporal.PlainDate(2000, 5, 2);
+const date2 = new Temporal.PlainDate(2000, 5, 2, calendar);
+
+[
+  [date1, "2000-05-02[u-ca=iso8601]"],
+  [date2, "2000-05-02[u-ca=custom]"],
+].forEach(([date, expected]) => {
+  const result = date.toString({ calendarName: "always" });
+  assert.sameValue(result, expected, "expected " + expected);
+});

test/built-ins/Temporal/PlainDate/prototype/toString/calendarname-auto.js

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+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.protoype.tostring
+description: auto value for calendarName option
+features: [Temporal]
+---*/
+
+const calendar = {
+  toString() { return "custom"; }
+};
+const date1 = new Temporal.PlainDate(2000, 5, 2);
+const date2 = new Temporal.PlainDate(2000, 5, 2, calendar);
+
+[
+  [date1, "2000-05-02"],
+  [date2, "2000-05-02[u-ca=custom]"],
+].forEach(([date, expected]) => {
+  const result = date.toString({ calendarName: "auto" });
+  assert.sameValue(result, expected, "expected " + expected);
+});

test/built-ins/Temporal/PlainDate/prototype/toString/calendarname-never.js

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+// Copyright (C) 2021 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.protoype.tostring
+description: never value for calendarName option
+features: [Temporal]
+---*/
+
+const calendar = {
+  toString() { return "custom"; }
+};
+const date1 = new Temporal.PlainDate(2000, 5, 2);
+const date2 = new Temporal.PlainDate(2000, 5, 2, calendar);
+
+[
+  [date1, "2000-05-02"],
+  [date2, "2000-05-02"],
+].forEach(([date, expected]) => {
+  const result = date.toString({ calendarName: "never" });
+  assert.sameValue(result, expected, "expected " + expected);
+});