Add more calendar: 'gregory' coverage to Test262 tests and/or demitasse tests

[justingrant]

This issue follows up on #1768 where @ptomato had a good idea: use existing ISO tests to cover non-ISO code by using calendar: 'gregory' which returns the same results (if year > 1) but executes the non-ISO polyfill code.

For datemath.mjs tests, using both calendars was super-easy. But we have hundreds of other tests which could probably leverage the same trick, both in demitasse and Test262. This is a tracking issue for this suggestion.

Notes:

• From Extend datemath tests to cover 'gregory' calendar #1768 (comment):

When converting the tests to test262, we should ensure that the iso8601 tests are added to built-ins/Temporal/Calendar but the gregory tests are added to intl402/Temporal/Calendar in test262.

• Could adding gregory support to these tests be automated, either using codegen as a one-time thing and/or doing like datemath.mjs where both calendars are used?

[ptomato]

I'll add this to tc39/test262#3002.