Add explicit test CI stage with coverage report artifacts

.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: golang:1.14
+  script:
+    - go get github.com/boumenot/gocover-cobertura
+    - go test -v -cover -coverprofile=cover.out ./...
+    - gocover-cobertura < cover.out > cover.xml
+  artifacts:
+    reports:
+      cobertura: cover.xml