[Temporal] Remove test/staging/Temporal/v8/calendar-months-in-year.js

And move tests into separate files for each type.

Author: catamorphism

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

@@ -9,3 +9,6 @@ features: [Temporal]
 
 const plainDate = new Temporal.PlainDate(1976, 11, 18);
 assert.sameValue(plainDate.monthsInYear, 12);
+assert.sameValue((new Temporal.PlainDate(1234, 7, 15)).monthsInYear, 12);
+assert.sameValue(Temporal.PlainDate.from('2019-03-18').monthsInYear, 12);
+assert.sameValue(Temporal.PlainDate.from('1234-03-18').monthsInYear, 12);

test/built-ins/Temporal/PlainDateTime/prototype/monthsInYear/basic.js

@@ -9,3 +9,4 @@ features: [Temporal]
 
 const datetime = new Temporal.PlainDateTime(1976, 11, 18, 15, 23, 30, 123, 456, 789, "iso8601");
 assert.sameValue(datetime.monthsInYear, 12, "check months in year information");
+assert.sameValue((new Temporal.PlainDateTime(1234, 8, 23, 5, 30, 13)).monthsInYear, 12);

test/built-ins/Temporal/ZonedDateTime/prototype/monthsInYear/basic.js

@@ -0,0 +1,12 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-get-temporal.zoneddatetime.prototype.monthsinyear
+description: Checking months in year for a "normal" case (non-undefined, non-boundary case, etc.)
+features: [Temporal]
+---*/
+
+const zdt = new Temporal.ZonedDateTime(217178610123456789n, "UTC");
+assert.sameValue(zdt.monthsInYear, 12, "check months in year information");
+assert.sameValue((new Temporal.ZonedDateTime(-23205637787000000000n, "UTC")).monthsInYear, 12);