build: set up a continuous integration pipeline for tests

Uses UiB’s shared workers to run the tests (make test) in continuous integration. This should help us increase the quality of contributions.

.gitlab-ci.yml

+image: python:3.9-buster
+
+variables:
+  PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+
+cache:
+  paths:
+    - .cache/pip
+    - venv/
+
+before_script:
+  - python -V
+
+test:
+  script:
+    - make test
+