Clang test failing

See e.g. https://gitlab.dune-project.org/extensions/dune-vectorclass/-/jobs/92863

There is a comment in .gitlab-ci.yml about why the clang tests are allowed to fail, but the comment is unclear whether we are ignoring a problem that we should fix when time permits, or whether upstream simply has no clang support.

So, basically, either improve the comment or fix this.