Add extensive tests for new Dir methods with platform-specific behavior

Introduce modular, comprehensive test cases for `Dir` methods, covering
various platform-specific paths. New tests ensure accurate handling of
environmental variables, relative paths, platform defaults, and fallback
strategies:

- Add test suites for `bin_home`, `cache_home`, `config_home`,
  `data_home`, `desktop`, `documents`, `downloads`, `fonts`, `home`,
  `music`, `pictures`, `preferences`, `publicshare`, `runtime`,
  `state_home`, and `templates`.
- Validate correct behavior for both Windows and Unix environments.
- Utilize `temp_env` for scoped environment variable handling.
- Ensure fallbacks to platform-specific directory standards when
  variables are unset or paths are invalid.

Refactor existing tests to match the added modular structure.

Author: aaronmallen