Exclude tests from wheels

[cananoo]

Fixes#402.

Configure Hatchling to exclude the bundled test package from wheel builds.
Source distributions continue to include the test suite.

Validation:

• python -m pytest -q -p no:ruff referencing/tests (503 passed, 132 xfailed, 100% coverage)
• python -m build --wheel --sdist --no-isolation
• Verified that the wheel omits referencing/tests while the sdist retains it
• ruff check referencing noxfile.py
• git diff --check

I used OpenAI Codex to help prepare this change; the patch and validation results were reviewed before submission.