Add more tests around DateDuration
From #7012:
Add tests and potentially fix bugs involving:
- [ ] Add more tests on algorithm behavior, such as Hebrew tests in the backwards direction
- [ ] Add more tests on constrain/reject behavior and edge cases (constrain should never fail)
Why is this 2.1 blocking? I thought we're not going to release date arithmetic?
Assessment for 2.1: Does not block milestone since it's pure testing.
Also this isn't stable.
Preemptively 2.2-blocking since this is not stable
Triage: Remains in the 2.2 milestone unless we decide that arithmetic is no longer a headline release item.
We should be careful writing tests for behavior involving https://github.com/tc39/proposal-intl-era-monthcode/issues/96, but other tests can be written.