test(format/date): remove duplicate cases and expand full-date boundary coverage

[AcEKaycgR]

Applies to: draft-04, draft-07, draft/2019-09, draft/2020-12

Removes 4 duplicate cases and adds 13 new cases across two related character-level categories: string-level boundaries and year-field format.

Removed:

• "2020-13-01" duplicates "1998-13-01"
• "1998-04-31" duplicates "2020-04-31"
• "2021-02-29" and "2020-02-29" in the per-month block duplicate the standalone leap year cases

Added (string-level boundaries):

• Empty string
• Embedded whitespace between year and month
• Trailing character, trailing Z, and space-separated datetime suffix

Added (year-field format):

• Valid four-digit year 0001 (lower bound)
• Two-digit, three-digit, and five-digit year
• Positive and negative sign prefix on year
• Non-ASCII Bengali digit in year field
• Alphabetic characters in year field

@jviotti @jdesrosiers @karenetheridge Ready for review. I've grouped these two character-level categories (boundaries and year-format) into this single PR.

[jviotti]

Maybe let's split the removing of duplicates and the addition of the new tests as separate PRs? Will be easier to review

[AcEKaycgR]

@jviotti That makes total sense. To make sure I follow:

• Should I update this PR to only include the removals, and then open a second PR for the additions?
• Or would you prefer I close this one and open two entirely fresh PRs?

[jviotti]

As you wish!