WIP: Add Tests Controller

Part of #218723 (closed)

What does this MR do?

This is a POC MR, where I'm exploring how we could load individual test suite data for a given build.

It includes the following items:

• Adds TestsController class
• Adds new routes tests.json and tests/:suite-name.json
• Adds new TestSuiteSerializer

As suggested by @ayufan we might have an opportunity to create our own tests resource and create a new controller to handle this logic.

Sure! Maybe we go as you did: model everything with separate endpoint to which frontend would switch? This does make me thing that this would be great if this would be separate controller under /pipelines/?

• badge + all test suites with counters: /pipelines/tests.json
• fetch details of a given test suite: /pipelines/tests/<suite-name>.json

Please only review the last commit, the rest has been extracted to a separate MR 🙏

Screenshots

endpoint	JSON response
merge_requests/:id/pipelines/:id/tests.json
merge_requests/:id/pipelines/:id/tests/:suite-name.json

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