Restart workers periodically to prevent memory accumulation (#4666)

I'm pretty sure the reference test generation working functions are
accumulating memory and this is why the runner is running out of memory.
I tested locally and found that this did make a difference. I will merge
this & see if it solves our issue.

I think switching to pytest might solve the underlying issue, not sure.
I believe this issue popped up now because etan's PR added many new SSZ
tests and that pushed it over the limit.

Author: jtraglia

tests/core/pyspec/eth2spec/gen_helpers/gen_base/gen_runner.py

@@ -284,7 +284,8 @@ def display_active_tests(active_tests, total_tasks, completed, skipped, width):
                         skipped.value += 1
                     completed.value += 1
             else:
-                pool = Pool(processes=args.threads)
+                # Restart workers periodically to prevent memory accumulation
+                pool = Pool(processes=args.threads, maxtasksperchild=100)
                 try:
                     for result in pool.uimap(worker_function, inputs):
                         if result == "skipped":