[libomptarget] Align test code with runtime/

This change allows setting LIBOMPTARGET_LLVM_LIT_EXECUTABLE and
LIBOMPTARGET_FILECHECK_EXECUTABLE as full path. It also honors
OPENMP_LLVM_TOOLS_DIR which is meant as a common configuration
for both libomp and libomptarget.

Maybe this should be done in a common CMake module, but I'm no expert here.

Differential Revision: https://reviews.llvm.org/D29172

llvm-svn: 294284