Python PostCommits produce too many spanner databases

There are ~30 Python PostCommit tests that create and destroy a Spanner Database. Each running across 3 PostCommits. We should reduce the number of tests that do this.

A couple of avenues:
1. Only run Spanner ITs in one Python version's PostCommit.
2. Verify more logic in spannerio_test.py (as opposed to `spannerio_\*_it_test.py`), which uses a mock rather than prod Spanner. We can still have a few ITs, but we don't need to verify *everything* there.

Imported from Jira [BEAM-13373](https://issues.apache.org/jira/browse/BEAM-13373). Original Jira may contain additional context.
Reported by: bhulette.
This issue has child subcomponents which were not migrated over. See the original Jira for more information.