storage: use testutil.NewRand in test
storage: use testutil.NewRand in test

Change-Id: Ic037439755862cf291d4ab9d292fb175f80bdc33
Reviewed-on: https://code-review.googlesource.com/30030
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go