barrier: fix race in test-code

The barrier_wait_next_twice* test-cases run:
  Parent:                             Child:
    set_alarm(10)                       sleep_for(1);
    ...                                 set_alarm(1);
    sleep_for(2)                        ...

Therefore, the parent exits after 2+ periods, the client's alarm fires
after 2+ periods. This race turns out to be lost by the child on other
machines, so avoid it by increasing the parent's sleep-interval to 4. This
way, the client has 2 periods to run the barrier test, which is far more
than enough.