posixtestsuite hangs whole Windows OS when run on system with large number of cores

Trying to run posixtestsuite on Windows to figure out why PR #12833 does not pass there, I notice that the suite spawns #numcores threads for each #numcores thread on the system, leading to 64*64=4096 python subprocesses running on my Threadripper system. This "fork bomb" causes Windows OS to become unstable.

Sometimes `taskkill /f /im python.exe` is able to recover, other times it can partially recover, and sometimes it just hangs as well. All other processes become unresponsive, and one has to hard reset the system.