cmake: clarify test parallelization #156

Job	Run time
heffte (cmake, BUILTIN, cpu_intel)	1m 21s
heffte (cmake, FFTW, cpu_intel)	1m 55s
heffte (cmake, MKL, cpu_intel)	1m 50s
heffte (spack, BUILTIN, cpu_intel)	16m 7s
heffte (spack, FFTW, cpu_intel)	16m 19s
heffte (spack, MKL, cpu_intel)	16m 24s
heffte (cmake, ONEAPI, gpu_intel)	1m 34s
heffte (cmake, CUDA, gpu_nvidia)	7m 42s
heffte (spack, CUDA, gpu_nvidia)	18m 45s
heffte (cmake, ROCM, gpu_amd)	40s
	1h 22m 37s

Provide feedback