CMAKE: enable TESTING_LIBRARY when BENCHMARKS
Currently building Benchmarks without VTKm_ENABLE_TESTING=ON results in failure. We already have a way to handle this with VTKm_ENABLE_TESTING_LIBRARY, we just need to add the dependency to VTKm_ENABLE_BENCHMARKS so that:
(VTKm_ENABLE_TESTING OR VTKm_ENABLE_BENCHMARKS) -> VTKm_ENABLE_TESTING_LIBRARY