Setup gitlab CI

This setups a simple CI that runs the tests and builds the pdf artifacts on any branch or MR.

Note that the tests are temporarily reverted to directly include the main IR calcul (necessary to make them work properly until https://github.com/CatalaLang/catala/issues/485 is resolved) ; a few quirks are also fixed (disabling warnings, fixing a typo)

The CI uses a pre-built Docker image with Catala pre-installed (see the setup instructions at https://gitlab.adullact.net/-/snippets/150 ; that image is hosted as catala on the gitlab registry of this repo). For now it's hand-generated but in time we should switch to an image automatically updated from the Catala repository and hosted on a separate registry.