Default to one thread per CPU, instead of 16 regardless of the number of CPUs
Default to one thread per CPU, instead of 16 regardless of the number of CPUs

This should be a better default for systems with few (or very many)
cores. Also, since we began starting the longest-running tests first,
there's no longer any need for extra parallelism just to get those
tests to start earlier.

google/gtest-parallel,ehlemur/gtest-parallel,greggomann/gtest-parallel,kwiberg/gtest-parallel,pbos/gtest-parallel,bbannier/gtest-parallel