CI for Test Suite

[fpjentzsch]

Adds base GitLab CI featuring:

• Nightly sync of finn-dev branch
• Build of Singularity image (only upon change of relevant files)
• FINN test suite

Another PR with CI for my benchmarking suite will follow.

TODO:

• Extend to full test suite
• Ensure all tests pass
• Test for both 2022.2 and 2024.2 (2024.1 for now as 2024.2 is not installed yet)
• Only run on PR
• Ensure correct Singularity image is always built and used
• Switch pipeline schedule back to dev once merged

[fpjentzsch]

Merging to dev even though I've not yet managed a run with 0 fails because it is still better than having no automatic tests on PRs at all.