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.