Add Gitlab CI configuration
Add a .gitlab-ci.yml file to automatically run tests on merge requests, the master branch, and tags.
Runs regression and upgrade tests on the latest ABCL, CCL, ECL, and SBCL on Debian using the Makefile based testing harness. The Lisp scripting based harness could be used (in addition to the Makefile based one) by specifying the
USE_ASDF_TOOLS environment variable when starting a pipeline.