In commit a653ca9504 (Tests: Update CUDA tests to work with Clang,
2020-03-27) some tests were conditioned using `CMAKE_CUDA_COMPILER_ID`.
That is not defined when configuring CMake itself, so it accidentally
turned off NVCC-specific CUDA tests altogether. Convert the conditions
to check `CMake_TEST_CUDA` for `Clang` instead. That option is added
explicitly to builds where we want the tests to run, so we can set it to
a value indicating the CUDA compiler vendor.
In commit a653ca9504 (Tests: Update CUDA tests to work with Clang,
2020-03-27) the NVCC-specific `CudaOnly.GPUDebugFlag` test was
accidentally broken by removing a space when appending `-G` to the CUDA
flags. This was covered by the test not running. Restore the space.
Fixes: #20727
