regrtest: Accept -jN% to target system load percentage, adaptively if possible #161204

As discussed at the core sprint, extend regrtest's -j option to accept a percentage instead of a fixed number of worker processes. Treat the given percentage as a target system load average (that is, on a system that returns 8 from os.process_cpu_count(), target 4.00 load avg with -j50%) and adaptively raise and lower the allowed worker count to best-effort target that load average.

PR to follow shortly.

Linked PRs

• gh-161204: Allow '-jN%' form of regrtest '-j' option #161205