Add CI to check XML validity

A simple CI check that checks the XMLs against their provided DTD.

The docker image described by the dockerfile included in this commit must still be compiled and pushed to this repository's docker registry for the CI to work.

This requires the following two commands to be executed by someone with the necessary access rights:

docker build -t registry.gitlab.com/sosy-lab/test-comp/bench-defs/test:validation - < test/Dockerfile.validation
docker push registry.gitlab.com/sosy-lab/test-comp/bench-defs/test