[!660] [classnametest] Skip if __cxa_demangle() is missing

Merge branch 'missing-cxa-demangle-skips-classnametest' into 'master'

ref:core/dune-common Partially addresses [#160]

See merge request [!660]

  [#160]: gitlab.dune-project.org/NoneNone/issues/160
  [!660]: gitlab.dune-project.org/core/dune-common/merge_requests/660