Testing with multiple threads

[vchuravy]

In conversation with @kpamnany and @d-netto we were wondering if PkgEval is stressting the scheduler and the upcoming parallel GC.

I think we would be fine to run with multiple threads (but only with a single CPU), and rr chaos mode. rr chaos mode uses up to 8 virtual CPUs so maybe we need -t auto to match that?

From my point of view other components such as BLAS can stay single-threaded.