prefetcher_test: Add test but leave it skipped because it sometimes flakes. Also add some helper methods.
prefetcher_test: Add test but leave it skipped because it sometimes flakes. Also add some helper methods.

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