Tweak CI and local integration tests

The purpose of this MR is to close #74 (closed) and #54 (closed).

This is a WIP, that does:

• Bumps relevant dependencies
• Introduces a separate test env
• Uses .env.{env} dotenv files, e.g. .env.test for env specific things (to avoid platform-specific sourceing)
• Adds documentation for running tests locally
• Adapts gitlab-ci.yml

I still need to test it in the CI itself, which seems to be a bit slow as the jobs are pending for a while. It should be complete today.