CI adaptations during external renderer merge

Roadmap from @tamarapu and @kiene for the external renderer merge related to CI:

• add BE comparison test on ext renderer branch (inactive at first)
  • will reuse existing renderer test, run both on target and source branch, compare outputs
  • use ext renderer testvectors for now, testv files would increase runtime
• after merge: activate BE comparison test in seperate MR
  • in the same course add it in the after-merge pipeline on main as well
• turn old test into "ext renderer smoke test" by using 1-sec testv files
• find reasonable set of testcases for BE comparison using testv files so that runtime is ok
  • similarly to #187 (closed) one can extend the version that runs on main after merges later