Coverage gap: Test what happens when PlainYearMonth's first day falls in ISO month -271821-03

[ptomato]

Thanks to @Josh-Cena I realized there's a weird case with PlainYearMonth that we don't have coverage for in test262. According to the spec, if there's any calendar where ISO date -271821-04-20 falls in a non-ISO month that starts on -271821-03-31 or earlier, Temporal.PlainDate.from('-271821-04-20').withCalendar(...).toPlainYearMonth() will throw a RangeError.

I haven't checked if any there's any actual calendar that has this problem. If there is, we should have coverage for it in test262.