Generate a single 'coverage' output line for tests

The numbers reported by Gitlab should now be accurate.

.gitlab-ci.yml

@@ -18,7 +18,12 @@ test:
   stage: test
   image: golang:1.15
   script:
-    - go test -v -cover -coverprofile=cover.out ./...
+    # The following monstruosity is required to generate a single coverage
+    # result statement for the entire package, but without generating build
+    # errors due to multiple definition of flags in cmd/ tools.
+    # See also https://github.com/golang/go/issues/27333
+    - go test -v -cover -coverpkg=$(go list ./... | grep -v /cmd/) -coverprofile=cover.out ./...
+    # Convert coverage output to XML for Gitlab integration.
     - (cd /tmp && go get github.com/boumenot/gocover-cobertura)
     - gocover-cobertura < cover.out > cover.xml
   artifacts: