WIP: Change build types and allow running CI outside of GROMACS project

This changes some of the current CI build types to represent our current pre- and post-submit builds better.

Jobs are reorganized between non-merge, merge and scheduled pipelines, with merges into our repository being set up to use the GROMACS project CI infrastructure to test e.g. accelerator support, while basic compilation and run tests are done using the normal Gitlab infrastructure.

Scheduled pipelines are also set to only use our servers.

Also splits up the current test script into a unit test part and a regression test part, allowing separation of concerns and making it possible to e.g. easily test mdrun only builds that can't run the usual regression tests.