Test className() even without demangling

Run the classnametest is now run two times:
- `classnametest-demangled`: skipped if `__cxa_demangle()` is missing
- `classnametest-fallback`: an exit-status of 1 (resulting from failed checks
  expecting demangling) is still considered successfull, though anything
  else (e.g. segfault) will fail the test.