Add pipeline run test to CI config

This is simply a question of adding the test call from the doc to .gitlab-ci.yml, so it could be done together with closing the below issues dealing with compiling a functional Snakefile and a set of test inputs and then documenting them:

• #6 (closed)
• #32 (closed)
• #24 (closed) (test call should go in here, too, so the easiest would be to add it to the GitLab CI/CD config at the same time)