Add test coverage artifacts to CI

.gitlab-ci.yml

 include: "https://git.autistici.org/ai3/build-deb/raw/master/ci-nextstable.yml"
+
+stages:
+  - test
+  - build_pkgsrc
+  - build_pkg
+  - upload_pkg
+
+build_pkgsrc:bullseye:
+  only:
+    - master
+
+build_pkg:bullseye:amd64:
+  only:
+    - master
+
+test:
+  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