Testcase module typed and formatted.

- Type hints for testcase module are completed.
- Type comments (to be compatible with Python 2) have been added to
  other parts of slapos.core only when necessary to achieve full type
  checking in the testcase module, with Pyright in strict mode.
- Docstrings added/completed for the testcase module.
- Source is formatted with ruff.

See merge request !693