Return suite_errors in test suite comparer

What does this MR do?

MR Widget section of #263457 (closed)

This is the API changes for /merge_requests/:id/test_reports.json.

Adds a new suite_errors under each suite entry.

Please note that this endpoint returns test suite comparison information. Thus, suite_errors will contain errors both from head_suite and base_suite. If head_suite or base_suite has no errors, the suite_errors will be null. Please see screenshot for sample response.

Screenshots (strongly suggested)

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team