Harden spanner system tests further. (#4374)
Harden spanner system tests further. (#4374)

Because of quota, we are sharing the instance we use to test CRUD of
databases: we therefore cannot rely on having only the expected databases
present, as other tests may be running simultaneously, or may have failed
in ways which left 'stray' databases in the instance.
jonparrott/gcloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,jonparrott/google-cloud-python,tswast/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tseaver/gcloud-python,jonparrott/gcloud-python,dhermes/gcloud-python,dhermes/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,dhermes/google-cloud-python,tseaver/google-cloud-python,dhermes/google-cloud-python,tseaver/gcloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python