gitlab-ci: Run go tests on multiple versions

This patch makes the GitLab CI config run go tests on Go 1.11 and the
latest Go release.

This is in addition to existing tests (which are renamed for clarity).

This replaces the main use case of Travis CI, which was to test on
multiple Go versions.

.gitlab-ci.yml

@@ -3,8 +3,31 @@ stages:
   - test
   - image
 
+# Go tests, on various Go versions.
+.golang_template: &golang
+  stage: test
+  before_script:
+    - useradd --create-home --user-group testing
+    - chown -R testing:testing $GOPATH/ .
+  script:
+    - su testing -c "go mod download"
+    - su testing -c "go get ./..."
+    - su testing -c "make all"
+    - su testing -c "go test ./..."
+    - su testing -c "go test -race ./..."
+
+golang_1.11:
+  <<: *golang
+  image: golang:1.11    # Version in Debian stable.
+  variables:
+    GO111MODULE: "on"
+
+golang_latest:
+  <<: *golang
+  image: golang:latest
+
 # Integration test, using the module versions from the repository.
-stable:
+integration_stable:
   stage: test
   image: docker:stable
   services:
@@ -16,7 +39,7 @@ stable:
     - docker run chasquid-test  make test
 
 # Integration test, using the latest module versions.
-latest:
+integration_latest:
   stage: test
   image: docker:stable
   services:
@@ -28,7 +51,7 @@ latest:
     - docker run chasquid-test  make test
 
 # Build docker image, upload to gitlab registry.
-build_gitlab:
+gitlab:
   stage: image
   image: docker:stable
   services:
@@ -40,7 +63,7 @@ build_gitlab:
     - docker push $CI_REGISTRY_IMAGE:$CI_BUILD_REF_NAME
 
 # Build docker image, upload to dockerhub registry.
-build_dockerhub:
+dockerhub:
   stage: image
   image: docker:stable
   services: