[GitLabCI] use venv in tests.

.gitlab/ci/test.gitlab-ci.yml

@@ -15,6 +15,7 @@ test:ubuntu_python:
     - docker
   image: n2d2-ci/ubuntu20.04/cpu:latest
   script:
+    - source venv/bin/activate
     - cd aidge_core
     - python3 -m pip list
     # Run on discovery all tests located in core/unit_tests/python and discard the stdout