Include test suite and tox configuration in source distributions

[saitakarcesme]

Fixes #359.

MANIFEST.in currently grafts tests, but the repository's suite and fixtures live under test. Correct that directory name and include tox.ini so downstream users can run the documented test command from an extracted source distribution.

Validation:

• Built before/after sdists: the original has no test directory or tox configuration; the fixed archive contains all 24 tracked test files and assets, plus tox.ini.
• Source archive size changes from 16,041 to 867,155 bytes (about 847 KiB including fixtures). The wheel still contains no test directory.
• Extracted the fixed sdist outside the checkout and ran its tox -e py313: 14 passed, 2 skipped.
• Repository tests: 14 passed, 2 skipped on Python 3.13 with libmagic 5.48; git diff --check passes.

AI disclosure: This contribution was prepared and tested by OpenAI Codex under the account owner's authorization.