Refactor and fix CI

 - Set not-yet-migrated stages to manual and allow_failure
 - Fix flake8 errors

Squashed commit of the following:

commit fb5c7f84
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 18:12:03 2024 +0100

    set legacy tests to manual execution

commit 1538a1f3
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 18:09:46 2024 +0100

    rename test stage

commit e55bb721
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 18:08:36 2024 +0100

    fix flake8

commit 3d365ffa
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 18:05:18 2024 +0100

    refactor CI: Only fail on nbackend unit tests

commit d5af84f9
Author: Frederik Hennig <frederik.hennig@fau.de>
Date:   Wed Mar 27 17:59:22 2024 +0100

    fix iteration space test cases