Resolve inline TODOs: missing analyzer tests and datafinder date validation

[SNO7E-G]

Hi team 👋
While exploring the codebase, I noticed a few places where maintainers had left
TODO comments requesting test coverage and input validation that were never
followed up on. I'd like to address all of them in a single focused PR.

What's currently missing

File	Issue
timesketch/lib/analyzers/ntfs_timestomp_test.py	Test body is self.assertEqual(True, True) — no real assertions
timesketch/lib/analyzers/domain_test.py	Only 22 lines, tests class instantiation only
timesketch/lib/analyzers/feature_extraction_test.py	Has # TODO: Add tests for the feature extraction with nothing below it
timesketch/lib/datafinder.py	Two instances of # TODO: Implement a check if this is a valid ISO formatted date

Proposed fix

• Replace the NTFS timestomp stub with real behavioral tests covering all branches
  of is_suspicious().
• Expand the domain analyzer test file with MockDataStore-based tests (empty stream,
  URL extraction, rare domains, CDN detection).
• Add unit tests for FeatureExtractionSketchPlugin — properties, empty plugin name,
  and unregistered plugin error handling.
• Implement ISO 8601 validation in set_start_date() / set_end_date() using
  datetime.fromisoformat(). Validation is non-breaking: invalid dates still get
  accepted with a logged warning to preserve backward compatibility.

Happy to discuss any of this before I open the PR. Thanks!