Adjust CI file include tests

What does this MR do and why?

This MR refactors CI file includes tests.

This is extracted from an MR (!108826 (merged)) because these can be merged as they are. In this MR, we are moving test files from mocking (allow(repository).to receive(:blob_data_at)) to real files (create_and_delete_files).

There is also a small refactoring that deletes an unused private method "project" in the file/artifact.rb file.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Indirectly related to #382531 (closed)