branchmark_test: add a warmup phase to trigger the large file fail
branchmark_test: add a warmup phase to trigger the large file fail

Issue: KBFS-1236

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