prefetcher_test: WIP regression test for KBFS-2499
prefetcher_test: WIP regression test for KBFS-2499

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