Add explicit CI test stage, with metadata

.gitlab-ci.yml

 include: "https://git.autistici.org/ai3/build-deb/raw/master/ci-backports.yml"
+
+run_tests:
+  stage: test
+  image: registry.git.autistici.org/ai3/docker/test/golang:master
+  script:
+    - run-go-test ./...
+  artifacts:
+    when: always
+    reports:
+      cobertura: cover.xml
+      junit: report.xml
+