kbfs_ops_test: make only one prefetcher, and shut it down
kbfs_ops_test: make only one prefetcher, and shut it down

`newBlockRetrievalQueue()` already makes a prefetcher, so use that one
for the test.  And make sure to shut it down after the test, to avoid
goroutine leaks.

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