Test gap for custom calendars

[Ms2ger]

          > Note that the same test gap seems to exist for custom calendars too, so a future PR will be needed to fill that gap.

FYI @ptomato: there may be a test gap for non-string custom calendar IDs. Where is the best place to record that this gap exists? Issue here in Test262 repo? In proposal-temporal repo? Somewhere else? I probably won't be able to PR that, but at least wanted to record that the gap (probably) exists.

Originally posted by @justingrant in #3881 (comment)