CI: streamline packet jobs definition

- Take advantage of `parallel:matrix` to make the jobs definition shorter
  and more readable.
- Remove helper scripts which are no longer needed
- Remove redundant indirection in the gitlab-ci pipelines definitions
  (only one user)

.gitlab-ci.yml

@@ -59,18 +59,6 @@ before_script:
     - pre-commit            # lint
     - vagrant-validate      # lint
 
-.testcases: &testcases
-  extends: .job-moderated
-  interruptible: true
-  before_script:
-    - update-alternatives --install /usr/bin/python python /usr/bin/python3 1
-    - ./tests/scripts/rebase.sh
-    - ./tests/scripts/testcases_prepare.sh
-  script:
-    - ./tests/scripts/testcases_run.sh
-  after_script:
-    - ./tests/scripts/testcases_cleanup.sh
-
 # For failfast, at least 1 job must be defined in .gitlab-ci.yml
 # Premoderated with manual actions
 ci-not-authorized: