spanner/spannertest: implement GetDatabase (#2802)
spanner/spannertest: implement GetDatabase (#2802)

The spannertest simulation does not represent multiple databases, and
the names of databases are ignored in requests, so this method merely
reflects the requested name back, along with the spannertest server
initialisation time.

Fixes #2789.
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go