test(format/date): assert hyphen literal separator compliance

[AcEKaycgR]

Applies to: draft-v1, draft-07, draft/2019-09, draft/2020-12

Adds 6 new cases covering separator character validation in the RFC 3339 Section 5.6 full-date grammar. This ensures that parsers strictly reject non-hyphen separators, mixed separators, and duplicated hyphens.

Added:

• Colon separators (2020:01:01)
• Dot separators (2020.01.01)
• Space separators (2020 01 01)
• Mixed slash and hyphen separators (2020-01/01)
• Duplicated first hyphen (2020--01-01)
• Duplicated second hyphen (2020-01--01)

@jviotti @jdesrosiers @karenetheridge Ready for review.

[jviotti]

It would be nice to see if any existing implementation does fail at this, just to get a better sense of the value of the tests as we've been very recently doing. But other than that, it looks spec correct!

[AcEKaycgR]

Thanks for the approval, @jviotti

Under Bowtie verification, strict engines like Go and Rust correctly reject these invalid separators. For engines like Python, C#, and Ruby, these tests establish necessary compliance boundaries when optional format checkers are explicitly enabled.