Immediate improvements to CI system

The initial CI setup has been merged to main, but some small TODOs that popped up during the setup are left, i.e.:

• Improve/fix the BE-check by changing back to the source branch commit before running self_test.py
• add -j flag to all make calls (and potentially already remove the .NOTPARRALEL target from the Makefile)
• add a new template .test-job-linux-needs-testv-dir so we only run the before_script step currently in .test-job-linux where it really is needed
• check why there are no logs archived from the smoke test job