multifold test stabilization

We've been seeing some nondeterministic test failures after the
multifold stuff went in, this patch is an attempt to make them stop
happening.

- bitcask_nifs: Prior to multifold, keydir_itr_many_update_test() and
  keydir_itr_many_out_of_date_test() tested different things.  They're
  also subject to a race condition that the test bots seem to hit all
  the time.  As of multifold, they become the same test, so rename,
  de-race, and deduplicate.

- bitcask: The fold_visits_frozen_test() and
  fold_visits_unfrozen_test() don't clean up their disk state between
  runs.  We've been seeing some timeouts, adding this cleanup to
  hopefully stop it from happening.