pubsub: use ip version agnostic hostname in testutil server
pubsub: use ip version agnostic hostname in testutil server

Fixes #1442

Change-Id: I374db9ff9da6b1b66388a7a4ff3b38813b57d8b3
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/41571
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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