AMD GPU support for the alpaka tests #51

The alpaka tests only run on CUDA at the moment. ALPAKA_BACKEND=hip exits with a not implemented error in test/CMakeLists.txt.

Thetest/alpaka/TestAlpakaCommon.h hardcodes PlatformCudaRt and DevCudaRt, includes cuda_runtime.h and calls cudaDeviceSynchronize() directly and the individual test files reference those CUDA types too, so nothing compiles for another backend as it stands.