block_retrieval_worker_test: Add logging to track down flake.
block_retrieval_worker_test: Add logging to track down flake.

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