Provide a setup for a faster feedback loop for acceptance tests locally

**Issue:**

The [setup process](https://github.com/googleapis/ruby-spanner/blob/main/google-cloud-spanner/acceptance/spanner_helper.rb) for spanner acceptance tests creates a new database and instance every time it is run. While this makes sense for the CI process, it is very slow during local development.

Database & instance creation is a slow process, so running even a single acceptance test takes 1-2 minutes locally.

**Recommended Solution:**

The setup process should accept an already existing database & instance, and run tests on them. After the run, it should avoid deleting the database & instance so the developer can reuse them again.

Start with google-cloud-spanner/acceptance/spanner_helper.rb and trace how acceptance tests create and clean up the instance and database. Determine how local runs can receive existing resources, then verify that repeated local runs reuse them without deleting them while CI behavior remains appropriate.

Written by the indexing model from the issue text.