Resolve #3487540 "Add gitlab template"

.gitlab-ci.yml

+# Use the DrupalCI templates.
+include:
+  - project: $_GITLAB_TEMPLATES_REPO
+    ref: $_GITLAB_TEMPLATES_REF
+    file:
+      - '/includes/include.drupalci.main.yml'
+      - '/includes/include.drupalci.variables.yml'
+      - '/includes/include.drupalci.workflows.yml'
+
+variables:
+  # Broaden test coverage.
+  OPT_IN_TEST_PREVIOUS_MAJOR: 1
+  OPT_IN_TEST_NEXT_MINOR: 1