Add unit tests for is_compliant_user_id_localpart

[guillemo12]

What I hope to achieve

Add missing unit test coverage for is_compliant_user_id_localpart in synapse/types/__init__.py.

How it addresses those goals

Added IsCompliantUserIdLocalpartTestCase in tests/test_types.py covering:

• Empty strings
• Compliant ASCII characters within the range U+0021 to U+007E (alphanumeric and valid symbols)
• Non-compliant characters (spaces, non-ASCII characters, control characters, tabs, newlines, and DEL \x7f).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)