Separate out test case generation from deployment

Rather than looping over branches (in included template from `pages`),
generate the test cases here, provide these as an artifact and deploy
in a separate job. This avoids the regeneration of pages that have
not changed.

Rather than using the page branch, use the apertis-test-cases-web as
part of a multi-project pipeline.

Whilst we are here, utilise an environment variable for the release to
make it easier to migrate this between releases.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

.gitlab-ci.yml

-image: docker-registry.apertis.org/apertis/apertis-19.03-testcases-builder
+variables:
+  release: v2019
+
+# We don't have v2019-testcases-builder in this repo, so let's use the latest for now.
+image: registry.gitlab.apertis.org/infrastructure/apertis-docker-images/v2021dev3-testcases-builder
 
 test-renderer:
   stage: test
   script:
     - python3 -m unittest discover -v
 
-test:
+render-pages:
   stage: test
   script:
-    - ./atc test-cases/ -d test/ --index-page
+    - ./atc -d ${release}/ --index-page test-cases/
   artifacts:
     paths:
-      - test
-  except:
-    - apertis/v2019
+      - ${release}
 
-pages:
+deploy-pages:
   stage: deploy
-  script:
-    - ./atc test-cases/ -d public/ --index-page
-  artifacts:
-    paths:
-      - public
-  only:
-    - apertis/v2019
+  trigger:
+    project: tests/apertis-test-cases-web
+    branch: main