Tests: Restore NVCC-specific CUDA tests

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

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