Use 1 test thread per CPU to avoid thread starvation on dual core CPUs

I guess 1 thread per CPU might work better than "6 threads".

On my dual-core macbook late 2008 (2.4Ghz core 2 duo, 2 cores, 2 threads total)  testing with 6 thread takes longer than with 1 per cpu: 9 minutes vs 5 minutes.