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

[SandeepTuniki]

Issue:

The setup process 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.

[SandeepTuniki]

Adding some data based on my benchmarking:

There are 3 parts to the setup process:

1. Create database & instance (through DB client)
2. Create pg database & run DDL queries (through DB admin)
3. Cleanup resources (after tests are run)

Below are the average run-times of each of the steps:

#	Step	Run time (avg.) (in sec.)
1	Create DB & instance	25.6
2	Create PG DB & run DDL queries	65.15
3	Cleanup resources	9.14
	Total time	99.89