feat: add unit tests for cpio and compression modules

[axelfrache]

Add unit tests for cpio and compression modules

Description

Adds comprehensive unit tests for the cpio.rs and compress.rs modules to improve test coverage and ensure the initramfs generation produces valid, bootable archives.

How Has This Been Tested?

All tests pass locally:

cargo test
# test result: ok. 16 passed; 0 failed

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[damien-mathieu1]

LGTM