[libomptarget-nvptx] Add testing infrastructure

This patch also introduces testing for libomptarget-nvptx
which has been missing until now. I propose to add tests for
all bugs that are fixed in the future.
The target check-libomptarget-nvptx is not run by default because
 - we can't determine if there is a GPU plugged into the system.
 - it will require the latest Clang compiler. Keeping compatibility
   with older releases would prevent testing newer code generation
   developed in trunk.

Differential Revision: https://reviews.llvm.org/D51687

llvm-svn: 343324