Fix flaky gitlab migration project import specs

Summary

Fixes flakiness in the gitlab migration project import specs by addressing two issues:

• Removed created_at from Project#comparable: This field always differs between source and imported projects, causing intermittent equality failures.
• Replaced opaque eq(source_project) with explicit attribute assertions: Asserts on name, path, and description individually, which produces clearer failure messages and avoids brittleness from unrelated comparable field changes.

Related issue

https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/24008