test: fix flaky multiprocessing tests

Tests are failing because we expect to see three different pids, but
only get two.  Is that because the work is being completed too quickly?

Author: nedbat

Diff for: tests/test_concurrency.py

@@ -336,8 +336,8 @@ def work(x):
     def process_worker_main(args):
         # Need to pause, or the tasks go too quickly, and some processes
         # in the pool don't get any work, and then don't record data.
-        time.sleep(0.02)
         ret = work(*args)
+        time.sleep(0.1)
         return os.getpid(), ret
 
     if __name__ == "__main__":      # pragma: no branch