[mix][library][test] refactor mix_test to use mix_library compilation artifacts and use a mix_env environment for library compilation

[arikaro-disc]

Refactored mix_test to depend on mix_library for compilation artifacts, eliminating duplicate compilation. Added mix_env parameter to mix_library to support test/prod/dev Mix environments. Cleaned up mix_library implementation with simplified priv file handling

• mix_test architecture: Now takes a lib attribute pointing to a mix_library target compiled with mix_env="test", reusing its compilation outputs instead of compiling independently
• mix_library enhancements: Supports configurable Mix environments via mix_env parameter, with validation in mix_test to ensure test libraries are built with mix_env="test"

[arikaro-disc]

Gave this an initial stab. Tear it apart, we can find time to discuss implementation/adjustments when you have breathing room.