Fix test error for non-CUDA machines

[fgittins]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This is a small fix. I've added a (second) CUDA.functional() if statement to the CUDA tests, so the test passes on machines that don't have an NVIDIA GPU installed or CUDA drivers.