pages: Test build non-master branches

Run the testcase renderer on merge requests and branches so we can catch some
failures early.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

 image: docker-registry.apertis.org/apertis/apertis-19.03-testcases-builder
 
+test:
+  stage: test
+  script:
+    - ./atc test-cases/ -d test/ --index-page
+  artifacts:
+    paths:
+      - test
+  except:
+    - master
+
 pages:
   stage: deploy
   script: