Rename TestReports to TestReport

What does this MR do and why?

Refactoring to rename the class.

Similar to !89099 (merged).

A TestReport represents a single report for code testing as a result of all jobs in a pipeline. There may be multiple jobs creating tests related job artifacts, but in aggregate, there is only one test report.

Screenshots or screen recordings

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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.

Related to #363302