Run tests faster when there are multiple CPUs.

The big problem here was that rr was pinning all tracees (and threads thereof) to CPU 0.  Not so friendly to -j unit tests ;).  I had a solution that randomly assigned tracess to CPUs, but it turned out that letting linux handle CPU pseudo-pinning was ~12% faster than randomly assigning.  That's good, since it's what I would expect from a decent scheduler.  So in light of that, this patch rips out the setaffinity code.

I'm testing on a 4-core machine with HT (8 HW threads).  Before this change, in a VM allocated 1 physical CPU, `make check` took ~250s.  After this change, in the same VM allocated 8 "physical" CPUs , `make check` is ~31s.  In the same 8-HW-thread VM, without this change, `make check` took around 149s.