kbfs_cr_test: sync a user before validating its GetDirChildren()
kbfs_cr_test: sync a user before validating its GetDirChildren()

Issue: #725

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