kbfs_ops_concur_test: make a few tests more timeout-friendly
kbfs_ops_concur_test: make a few tests more timeout-friendly

Suggested by akalin-keybase.

Issue: #650

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