[Temporal] Add coverage for month before earliest supported month

[catamorphism]

For PlainYearMonth.from with non-ISO calendars, add test coverage for the cases where the month is one month before the earliest supported month, and the month is one month after the latest supported month (an exception should be thrown).

Closes tc39/proposal-temporal#3082

[Manishearth]

I don't think this test is correct for Coptic, and I think the polyfill is wrong here.

Temporal.PlainDate.from({ calendar: "coptic", year: 275471, era: "am", eraYear: 275471, month: 5, day: 1); gives 275760-08-23[u-ca=coptic] in the polyfill. 275760-09-13 is the max ISO date.

The coptic test tests Temporal.PlainYearMonth.from({ calendar: "coptic", year: 275471, era: "am", eraYear: 275471, month: 6, monthCode: "M06"});, which will of course produce a value after 275760-09-13. The corresponding date for day: 1 is 275760-09-22

But: CalendarYearMonthFromFields doesn't check for the ISO date being out of range, it only checks ISOYearMonthWithinLimits, which checks the month not the day. As far as I can understand it, this test does not match the spec.

cc @ptomato who might catch something I missed.

[ptomato]

Thanks for checking, I'll take a look tomorrow.

[ptomato]

@Manishearth I concur, looks like I made a wrong assumption when I wrote the issue description in tc39/proposal-temporal#3082

[catamorphism]

Created tc39/proposal-temporal#3251 to track the bug in the polyfill.