Test more date constructor patterns in a roundtrip tests

[Manishearth]

Related: #7852

We caused a pretty big regression in Date::try_from_fields, see #7847. It is fixed now, but we should have better tests.

icu_calendar has many ways of constructing dates from fields

• Date::try_from_fields
  • This API can be called with a dozen different input sets: (era + year, extended year, both) + (month type, ordinal month, both, maybe also month codes)
• Date::try_new() (+ legacy Date::try_from_codes())
• Date::try_new_foobar()

We should test that all of these roundtrip, especially Date::try_from_fields().

We may also wish to test that we get InconsistentYear and InconsistentMonth when we supply inconsistent values

[Manishearth]

cc @robertbastian @sffc