test(#6092): Add Pixi environment tests

- Introduced PixiCacheTest and PixiConfigTest classes to validate Pixi environment functionality.
- Added tests for TOML and lock file detection, environment prefix path creation, and command execution handling.
- Implemented checks for configuration options and cache directory retrieval in PixiConfig.

This commit enhances the test coverage for Pixi environment management, ensuring robust functionality within the Nextflow framework.

Signed-off-by: Edmund Miller <[email protected]>

Author: edmundmiller