kbfs_cr_test: fix Fatal call, suggested by @akalin-keybase
kbfs_cr_test: fix Fatal call, suggested by @akalin-keybase

Issue: #882

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