On Thu 5 Mar 2020, Cloud Spanner announced support for FOREIGN KEYS as per https://cloud.google.com/spanner/docs/release-notes and https://cloud.google.com/spanner/docs/foreign-keys/overview

We need to add support for FOREIGN KEYS now, perhaps per https://github.com/orijtech/django-spanner/blob/189b71de3ab868c4924708396086843db6a6e506/spanner/django/features.py#L13

and then enable the tests that assumed no FOREIGN KEY support.