Minimize RuleRunner parallelism. (#12206)
Minimize RuleRunner parallelism. (#12206)

Minimize PyExecutor core_threads / max_threads by experiment while
keeping IT test time the same or better. The goal here is to minimize
contention to maximize throughput in CI and hopefully reduce the
incidence of test timeouts.

A similar experiment pants_integrationtest.py proved to always make
things worse.
pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants