gitlab-ci: Run unit tests

We have them, let's use them as much as possible.

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

.gitlab-ci.yml

 image: docker-registry.apertis.org/apertis/apertis-19.03-testcases-builder
 
+test-renderer:
+  stage: test
+  script:
+    - python3 -m unittest discover -v
+
 test:
   stage: test
   script: