[SE] Fix config bug with CUDA tests

Summary:
It turns out CMake errors out if a processed directory contains source
files that are not used. This was causing an error with the CUDATest.cpp
file when configuring StreamExecutor with the CUDA platform disabled.

Moving CUDATest.cpp to its own directory fixes this problem.

Reviewers: jlebar, jprice

Subscribers: beanz, mgorny, jlebar, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24618

llvm-svn: 281654