Improve specs for Ci::ProcessBuildService

What does this MR do?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52279

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [- ] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides