Define explicit :subject on restorer spec

What does this MR do?

Set subject explicitly in particular project_tree_restorer context.
From the follow-up #37357 (closed)

I also tried to "unify" all other specs in the file following a similar logic, but it didn't work well:

• we have two different approaches mixed in the same spec file: for complex json, we restore only once (performance concerns), while for other jsons we restore the project for each spec
• for particular examples, we use some visibility levels stubs performed before the restoration, unifying which adds complexity instead of reducing it

I went with the smallest change to address the follow-up here.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A