Add explicit coverage for 'row_data._retry_read_rows_exception'. (#6364)
Add explicit coverage for 'row_data._retry_read_rows_exception'. (#6364)

Fixes broken coverage on 'master'.
googleapis/google-cloud-python,dhermes/gcloud-python,tseaver/google-cloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,GoogleCloudPlatform/gcloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,jonparrott/gcloud-python,jonparrott/google-cloud-python,GoogleCloudPlatform/gcloud-python,dhermes/gcloud-python,tseaver/google-cloud-python