Ensure all Python multiprocessing tests have timeouts

[pentschev]

Ensure all Python multiprocessing tests timeout during join and get terminated properly, appropriately raising an error if the subprocess failed to terminate cleanly.

To simplify joining of multiple processes, a new testing function join_processes was added, where all processes in the list will be joined with a common timeout, if the total time elapsed is longer than the timeout the process will still be joined but wait will be non-positive, meaning join returns immediately and the process is later terminated with terminate_process.

[wence-]

Looks fine other than one question about potential for negative timeouts?

[wence-]

question: What happens if the timeout you pass to join is negative? I suspect that can occur since joining a process and getting to the next iteration can take a non-zero amount of time.

[pentschev]

Yes, I tested that and it behaves the same as timeout=0, essentially returns immediately, although the documentation only mentions positive timeouts (to me implies any non-positive number is indeed expected to return immediately), do you think we should attempt to make the value always non-negative in case this is not documented because it's undefined behavior/implementation-dependent?