Some unit tests depend on the particular version of clang.

I.e. 2db8e28 is necessary since clang 7.x in an old ubuntu container exposes different behaviour than the clang 11.x shipped with fedora 33. This might also be partly due to the default env flags.

TODO:

• investigate the true reason for the different behaviour
  • compare env vars
  • test behviour with identical env vars on different clang version