Add tests documenting date range formatter fallback issues

[Manishearth]

#8359 documents an issue with und data fallback in range formatting

This shows that we have that issue, and which locales we have it in, so that the work to fix it has tests to update.

(It's more convincing when you have currently-failing tests to fix, rather than when you show some passing tests and say "trust me these cases were broken before")

Changelog (N/A)

[sffc]

Issue: I think we probably shouldn't land std::panic::catch_unwind. Also, note that the panic is a debug_assert in impl Writeable.

Suggestion: avoid std::panic::catch_unwind by using a power-user API. I think there are 2 ways in the single formatter code path:

1. Use format_unchecked -> FormattedDateTimeUnchecked, which returns a TryWriteable, which you can inspect for an error
2. Use format -> FormattedDateTime -> pattern and DateTimeNames::from_formatter to manually format the pattern

This might need new APIs for DateTimeRangeFormatter, but those are APIs we would want anyway for graduation.

[Manishearth]

format_unchecked seems like a good call

[Manishearth]

We don't have format_unchecked, are you saying I should add it? I'd rather not go that far.

[Manishearth]

The thing to do here is instead to use should_panic and only run it in debug mode

[Manishearth]

Made an attempt.

[sffc]

We don't have format_unchecked, are you saying I should add it? I'd rather not go that far.

I said in my first comment,

This might need new APIs for DateTimeRangeFormatter, but those are APIs we would want anyway for graduation.

but I'm okay with should_panic.

[sffc]

dart CI:

yq v4.53.3 [Approved]
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