Tezt: Support junit.xml format

Make Tezt support the junit.xml test result report format. This allows for integration in the CI, as is done for pytest suite in !2742 (merged).

• Issue to add it in Alcotest https://github.com/mirage/alcotest/issues/130
• Docs on this feature in gitlab: https://docs.gitlab.com/ee/ci/pipelines/job_artifacts.html#artifactsreportsjunit.

Clement's comment gives some screen shots showing off this features.