Adds test coverage for job artifact statistics

What does this MR do and why?

During investigation of #368326 (closed) I noticed that the test coverage for Ci::JobArtifacts::DestroyBatchService did not confirm the expected value when there are multiple artifacts per project being destroyed.

This MR modifies the spec slightly to cover that scenario.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.