[libomptarget][cuda] Only run tests when sure there is cuda available

[libomptarget][cuda] Only run tests when sure there is cuda available

Prior to D95155, building the cuda plugin implied cuda was installed locally.
With that change, every machine can build a cuda plugin, but they won't all have
cuda and/or an nvptx card installed locally.

This change enables the nvptx tests when either:
- libcuda is present
- the user has forced use of the dlopen stub

The default case when there is no cuda detected will no longer attempt to
run the tests on nvptx hardware, as was the case before D95155.

Reviewed By: jdoerfert, ronlieb

Differential Revision: https://reviews.llvm.org/D95467