Harden 'test_list_entry_with_unregistered' against 429 errors. (#6181)
Harden 'test_list_entry_with_unregistered' against 429 errors. (#6181)

Closes #5759.
jonparrott/gcloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,googleapis/google-cloud-python,tswast/google-cloud-python,jonparrott/google-cloud-python,dhermes/google-cloud-python,dhermes/gcloud-python