prefetcher_test: Add test for multi-level directory prefetching.
prefetcher_test: Add test for multi-level directory prefetching.

As intended, this new test hangs without the subsequent commit.

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client