Re-enable JUnit reports
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30254 the JUnit reports for our test suites were disabled because of gitlab-com/gl-infra/production#928 (closed). We need to clarify what's necessary to enable them again, so we can dogfood the feature (https://docs.gitlab.com/ee/ci/junit_test_reports.html).